diff --git a/id_configurator/config.py b/id_configurator/config.py
index c964669..aed9f19 100644
--- a/id_configurator/config.py
+++ b/id_configurator/config.py
@@ -46,6 +46,10 @@ class AptSource(BaseModel):
     - `signed_by_file`: the *path* to a keyring already expected to exist on
       the target system, written to the rendered `Signed-By` field verbatim.
     If both are set, `signed_by` takes precedence.
+
+    `uri_to_replace`, if set, is an fnmatch pattern: this source is only
+    applied if it matches at least one URI already present in the machine's
+    existing apt configuration; otherwise it is skipped entirely.
     """
 
     model_config = ConfigDict(populate_by_name=True)
@@ -75,6 +79,7 @@ class AptSource(BaseModel):
     check_date: bool | None = Field(default=None, alias="check-date")
     date_max_future: str | None = Field(default=None, alias="date-max-future")
     description: str | None = None
+    uri_to_replace: str | None = Field(default=None, alias="uri-to-replace")
 
     def to_deb822(
         self,
@@ -143,11 +148,17 @@ class AptConfig(BaseModel):
 
 
 class ApkRepository(BaseModel):
-    """An Alpine APK repository (see /etc/apk/repositories), e.g. a URL ending in /main or /community."""
+    """An Alpine APK repository (see /etc/apk/repositories), e.g. a URL ending in /main or /community.
+
+    `url_to_replace`, if set, is an fnmatch pattern: this repository is only
+    applied if it matches at least one URL already present in the machine's
+    existing apk configuration; otherwise it is skipped entirely.
+    """
 
     name: str
     url: str
     key: str | None = None
+    url_to_replace: str | None = None
 
 
 class ApkConfig(BaseModel):
@@ -155,10 +166,19 @@ class ApkConfig(BaseModel):
 
 
 class YumRepository(BaseModel):
+    """A YUM/DNF repository (see /etc/yum.repos.d/*.repo).
+
+    `index_url_to_replace`, if set, is an fnmatch pattern: this repository is
+    only applied if it matches at least one baseurl already present in the
+    machine's existing yum/dnf configuration; otherwise it is skipped
+    entirely.
+    """
+
     model_config = ConfigDict(populate_by_name=True)
     name: str
     index_url: str = Field(alias="index-url")
     gpg_url: str | None = Field(default=None, alias="gpg-url")
+    index_url_to_replace: str | None = Field(default=None, alias="index-url-to-replace")
 
 
 class YumConfig(BaseModel):
diff --git a/id_configurator/modules/apk.py b/id_configurator/modules/apk.py
index 2469db9..3cd2092 100644
--- a/id_configurator/modules/apk.py
+++ b/id_configurator/modules/apk.py
@@ -1,18 +1,27 @@
 """Detect existing APK (Alpine) repositories and configure new ones.
 
 If `<src>/etc/apk` exists, the existing `/etc/apk/repositories` file (if any)
-is copied into `<dst>` with every active line disabled (commented out), and
-the repositories declared in the configuration are appended as new, active
-lines. Public keys provided alongside a repository are written to
+is copied into `<dst>`, and the repositories declared in the configuration
+are appended as new, active lines. An existing active line is only disabled
+(commented out) if it matches the `url_to_replace` pattern of one of the
+configured repositories; lines matching no such pattern are copied through
+unchanged. Public keys provided alongside a repository are written to
 `<dst>/etc/apk/keys/`, named after their content so that repositories
 sharing the same key (a common case for official Alpine mirrors) only
 produce a single key file.
+
+A repository's `url_to_replace`, if set, is an fnmatch pattern checked
+against the URLs already active in /etc/apk/repositories: the repository
+is only written out if the pattern matches at least one of them, and
+skipped entirely otherwise.
 """
 
 from __future__ import annotations
 
+import fnmatch
 import hashlib
 import logging
+from collections.abc import Sequence
 from pathlib import Path
 
 from id_configurator.context import Context
@@ -25,11 +34,18 @@ _REPOSITORIES = "etc/apk/repositories"
 _KEYS_DIR = "etc/apk/keys"
 
 
-def _disable_repositories_content(content: str) -> str:
-    lines = [
-        line if not line.strip() or line.strip().startswith("#") else f"# {line}"
-        for line in content.splitlines()
-    ]
+def _disable_repositories_content(content: str, patterns: Sequence[str]) -> str:
+    lines = []
+    for line in content.splitlines():
+        stripped = line.strip()
+        if (
+            stripped
+            and not stripped.startswith("#")
+            and any(fnmatch.fnmatch(stripped, pattern) for pattern in patterns)
+        ):
+            lines.append(f"# {line}")
+        else:
+            lines.append(line)
     return "\n".join(lines) + "\n"
 
 
@@ -37,6 +53,19 @@ def _key_filename(content: bytes) -> str:
     return f"{hashlib.sha256(content).hexdigest()[:16]}.rsa.pub"
 
 
+def _existing_urls(context: Context) -> set[str]:
+    """Collect the URLs already declared in /etc/apk/repositories on the
+    source filesystem (one active, non-comment line per URL)."""
+    existing = context.read_src_file(_REPOSITORIES)
+    if existing is None:
+        return set()
+    return {
+        stripped
+        for line in existing.decode("utf-8").splitlines()
+        if (stripped := line.strip()) and not stripped.startswith("#")
+    }
+
+
 @register
 class ApkModule(Module):
     name = "apk"
@@ -52,24 +81,46 @@ class ApkModule(Module):
         return None
 
     def apply(self, context: Context) -> None:
-        disabled = self._disabled_existing_repositories(context)
+        patterns = [
+            repository.url_to_replace
+            for repository in context.config.configurations.apk.repositories
+            if repository.url_to_replace is not None
+        ]
+        disabled = self._disabled_existing_repositories(context, patterns)
         configured = self._configured_repositories_lines(context)
         content = disabled + ("\n".join(configured) + "\n" if configured else "")
         if content:
             context.write_file(_REPOSITORIES, content.encode("utf-8"))
 
     @staticmethod
-    def _disabled_existing_repositories(context: Context) -> str:
+    def _disabled_existing_repositories(
+        context: Context, patterns: Sequence[str]
+    ) -> str:
         existing = context.read_src_file(_REPOSITORIES)
         if existing is None:
             return ""
-        logger.info("disabled existing apk repositories %s", _REPOSITORIES)
-        return _disable_repositories_content(existing.decode("utf-8"))
+        logger.info("copied existing apk repositories %s", _REPOSITORIES)
+        return _disable_repositories_content(existing.decode("utf-8"), patterns)
 
     @staticmethod
     def _configured_repositories_lines(context: Context) -> list[str]:
         lines = []
+        existing_urls: set[str] | None = None
         for repository in context.config.configurations.apk.repositories:
+            if repository.url_to_replace is not None:
+                if existing_urls is None:
+                    existing_urls = _existing_urls(context)
+                if not any(
+                    fnmatch.fnmatch(url, repository.url_to_replace)
+                    for url in existing_urls
+                ):
+                    logger.info(
+                        "skipped apk repository %s: url-to-replace %s matched "
+                        "no existing url",
+                        repository.name,
+                        repository.url_to_replace,
+                    )
+                    continue
             lines.append(repository.url)
             logger.info("added apk repository %s", repository.name)
             if repository.key:
diff --git a/id_configurator/modules/apt.py b/id_configurator/modules/apt.py
index 765b0d9..8f6fb0e 100644
--- a/id_configurator/modules/apt.py
+++ b/id_configurator/modules/apt.py
@@ -2,9 +2,13 @@
 
 If `<src>/etc/apt` exists, any source file already present there (classic
 one-line `.list` files or DEB822 `.sources` files, whether at the top level
-or under `sources.list.d/`) is copied into `<dst>/etc/apt`, disabled so it no
-longer takes effect, and the sources declared in the configuration are added
-as new, enabled `.sources` files.
+or under `sources.list.d/`) is copied into `<dst>/etc/apt`, and the sources
+declared in the configuration are added as new, enabled `.sources` files.
+
+An existing entry (a single `deb`/`deb-src` line, or a DEB822 stanza) is only
+disabled if its URI matches the `uri_to_replace` pattern of one of the
+configured sources; entries matching no such pattern are copied through
+unchanged, still enabled.
 
 Each source can be trusted two ways: `signed_by` holds the ASCII-armored
 content of a GPG public key, folded inline as a multi-line `Signed-By`
@@ -13,11 +17,18 @@ genuinely signed repository, so no `gpg` binary or separate keyring file
 is needed at all to apply this configuration). `signed_by_file` instead
 names a keyring already expected to exist on the target system, used
 verbatim as `Signed-By`; if both are given, `signed_by` takes precedence.
+
+A source's `uri_to_replace`, if set, is an fnmatch pattern checked against
+the URIs of the sources already present on the source filesystem: the
+source is only written out if the pattern matches at least one of them,
+and skipped entirely otherwise - even on an otherwise Debian-like machine.
 """
 
 from __future__ import annotations
 
+import fnmatch
 import logging
+from collections.abc import Sequence
 from pathlib import Path
 
 from id_configurator.context import Context
@@ -30,24 +41,100 @@ _SOURCES_LIST = "etc/apt/sources.list"
 _SOURCES_LIST_D = "etc/apt/sources.list.d"
 
 
-def _disable_list_content(content: str) -> str:
-    lines = [
-        line if not line.strip() or line.strip().startswith("#") else f"# {line}"
-        for line in content.splitlines()
-    ]
+def _uri_from_deb_line(stripped_line: str) -> str | None:
+    """Extract the URI from a single, already-stripped `deb`/`deb-src`
+    line (`deb [options] uri suite [component...]`), or `None` if the line
+    is not such a line."""
+    parts = stripped_line.split()
+    if not parts or parts[0] not in ("deb", "deb-src"):
+        return None
+    parts = parts[1:]
+    while parts and parts[0].startswith("["):
+        while parts and not parts[0].endswith("]"):
+            parts.pop(0)
+        if parts:
+            parts.pop(0)
+    return parts[0] if parts else None
+
+
+def _disable_list_content(content: str, patterns: Sequence[str]) -> str:
+    lines = []
+    for line in content.splitlines():
+        stripped = line.strip()
+        uri = (
+            None
+            if not stripped or stripped.startswith("#")
+            else _uri_from_deb_line(stripped)
+        )
+        if uri is not None and any(
+            fnmatch.fnmatch(uri, pattern) for pattern in patterns
+        ):
+            lines.append(f"# {line}")
+        else:
+            lines.append(line)
     return "\n".join(lines) + "\n"
 
 
-def _disable_deb822_content(content: str) -> str:
-    disabled_stanzas = []
+def _uris_from_deb822_stanza(stanza: str) -> set[str]:
+    uris: set[str] = set()
+    for line in stanza.splitlines():
+        if line.lower().startswith("uris:"):
+            uris.update(line.split(":", 1)[1].split())
+    return uris
+
+
+def _disable_deb822_content(content: str, patterns: Sequence[str]) -> str:
+    stanzas = []
     for stanza in content.split("\n\n"):
         if not stanza.strip():
             continue
         lines = stanza.splitlines()
-        new_lines = [line for line in lines if not line.lower().startswith("enabled:")]
-        new_lines.append("Enabled: no")
-        disabled_stanzas.append("\n".join(new_lines))
-    return "\n\n".join(disabled_stanzas) + "\n"
+        uris = _uris_from_deb822_stanza(stanza)
+        if any(fnmatch.fnmatch(uri, pattern) for uri in uris for pattern in patterns):
+            lines = [line for line in lines if not line.lower().startswith("enabled:")]
+            lines.append("Enabled: no")
+        stanzas.append("\n".join(lines))
+    return "\n\n".join(stanzas) + "\n"
+
+
+def _uris_from_list_content(content: str) -> set[str]:
+    """Extract the URIs declared in a classic one-line `.list` file
+    (`deb [options] uri suite [component...]`), skipping comments and any
+    bracketed option list."""
+    uris: set[str] = set()
+    for line in content.splitlines():
+        stripped = line.strip()
+        if not stripped or stripped.startswith("#"):
+            continue
+        uri = _uri_from_deb_line(stripped)
+        if uri is not None:
+            uris.add(uri)
+    return uris
+
+
+def _uris_from_deb822_content(content: str) -> set[str]:
+    """Extract the URIs declared in the `URIs:` field of each stanza of a
+    DEB822 `.sources` file."""
+    uris: set[str] = set()
+    for stanza in content.split("\n\n"):
+        uris.update(_uris_from_deb822_stanza(stanza))
+    return uris
+
+
+def _existing_uris(context: Context) -> set[str]:
+    """Collect every URI declared by the apt sources already present on the
+    source filesystem, whether in the top-level sources.list or under
+    sources.list.d/, in either the classic or the DEB822 format."""
+    uris: set[str] = set()
+    existing_top_level = context.read_src_file(_SOURCES_LIST)
+    if existing_top_level is not None:
+        uris.update(_uris_from_list_content(existing_top_level.decode("utf-8")))
+    for entry in context.iter_src_dir(_SOURCES_LIST_D):
+        if entry.suffix == ".sources":
+            uris.update(_uris_from_deb822_content(entry.read_text(encoding="utf-8")))
+        elif entry.suffix == ".list":
+            uris.update(_uris_from_list_content(entry.read_text(encoding="utf-8")))
+    return uris
 
 
 @register
@@ -65,32 +152,57 @@ class AptModule(Module):
         return None
 
     def apply(self, context: Context) -> None:
-        self._copy_existing_disabled(context)
+        patterns = [
+            source.uri_to_replace
+            for source in context.config.configurations.apt.sources
+            if source.uri_to_replace is not None
+        ]
+        self._copy_existing_disabled(context, patterns)
         self._write_configured_sources(context)
 
     @staticmethod
-    def _copy_existing_disabled(context: Context) -> None:
+    def _copy_existing_disabled(context: Context, patterns: Sequence[str]) -> None:
         existing_top_level = context.read_src_file(_SOURCES_LIST)
         if existing_top_level is not None:
-            disabled = _disable_list_content(existing_top_level.decode("utf-8"))
+            disabled = _disable_list_content(
+                existing_top_level.decode("utf-8"), patterns
+            )
             context.write_file(_SOURCES_LIST, disabled.encode("utf-8"))
-            logger.info("disabled existing apt source list %s", _SOURCES_LIST)
+            logger.info("copied existing apt source list %s", _SOURCES_LIST)
 
         for entry in context.iter_src_dir(_SOURCES_LIST_D):
             if entry.suffix == ".sources":
-                disabled = _disable_deb822_content(entry.read_text(encoding="utf-8"))
+                disabled = _disable_deb822_content(
+                    entry.read_text(encoding="utf-8"), patterns
+                )
             elif entry.suffix == ".list":
-                disabled = _disable_list_content(entry.read_text(encoding="utf-8"))
+                disabled = _disable_list_content(
+                    entry.read_text(encoding="utf-8"), patterns
+                )
             else:
                 continue
             context.write_file(
                 f"{_SOURCES_LIST_D}/{entry.name}", disabled.encode("utf-8")
             )
-            logger.info("disabled existing apt source %s", entry.name)
+            logger.info("copied existing apt source %s", entry.name)
 
     @staticmethod
     def _write_configured_sources(context: Context) -> None:
+        existing_uris: set[str] | None = None
         for source in context.config.configurations.apt.sources:
+            if source.uri_to_replace is not None:
+                if existing_uris is None:
+                    existing_uris = _existing_uris(context)
+                if not any(
+                    fnmatch.fnmatch(uri, source.uri_to_replace) for uri in existing_uris
+                ):
+                    logger.info(
+                        "skipped apt source %s: uri-to-replace %s matched no "
+                        "existing uri",
+                        source.name,
+                        source.uri_to_replace,
+                    )
+                    continue
             context.write_file(
                 f"{_SOURCES_LIST_D}/{sanitize_filename(source.name)}.sources",
                 source.to_deb822(
diff --git a/id_configurator/modules/yum.py b/id_configurator/modules/yum.py
index d0caa7a..0cd70e3 100644
--- a/id_configurator/modules/yum.py
+++ b/id_configurator/modules/yum.py
@@ -4,10 +4,13 @@ DNF is a drop-in successor to YUM and reads the exact same `.repo` files
 under `/etc/yum.repos.d/`, so a single module covers both.
 
 If `<src>/etc/yum.repos.d` exists, every `.repo` file already there is
-copied into `<dst>` with all of its sections disabled (`enabled=0`), before
-the repositories declared in the configuration are written as new, enabled
-`.repo` files (so a configured repository reusing an existing file name
-ends up enabled, taking precedence over the disabled copy). Each `.repo`
+copied into `<dst>`, before the repositories declared in the configuration
+are written as new, enabled `.repo` files (so a configured repository
+reusing an existing file name ends up enabled, taking precedence over the
+copied one). A section of an existing `.repo` file is only disabled
+(`enabled=0`) if its `baseurl`/`mirrorlist` matches the
+`index_url_to_replace` pattern of one of the configured repositories;
+sections matching no such pattern are copied through unchanged. Each `.repo`
 file is written referencing its `gpg-url` (used verbatim as `gpgkey=`,
 exactly as YUM/DNF expect: `https://`, or a `file://` path already present
 on the target system). Separately, each GPG key declared in
@@ -17,15 +20,22 @@ conventional location such keys live at on RPM-based systems. `gpg` is only
 used here to validate the key; the actual trust decision is left to
 YUM/DNF, which import it from that file the first time the repository is
 used (as `rpm --import` would).
+
+A repository's `index_url_to_replace`, if set, is an fnmatch pattern
+checked against the baseurl of every section of every existing `.repo`
+file: the repository is only written out if the pattern matches at least
+one of them, and skipped entirely otherwise.
 """
 
 from __future__ import annotations
 
 import configparser
+import fnmatch
 import logging
 import shutil
 import subprocess
 import tempfile
+from collections.abc import Sequence
 from io import StringIO
 
 from id_configurator.config import NamedContent, YumRepository
@@ -38,24 +48,46 @@ _REPOS_DIR = "etc/yum.repos.d"
 _RPM_GPG_DIR = "etc/pki/rpm-gpg"
 
 
-def _disable_repo_content(content: str) -> str:
+def _disable_repo_content(content: str, patterns: Sequence[str]) -> str:
     parser = configparser.ConfigParser(interpolation=None)
     parser.optionxform = str
     parser.read_string(content)
     for section in parser.sections():
-        parser[section]["enabled"] = "0"
+        urls = [parser[section].get(key) for key in ("baseurl", "mirrorlist")]
+        urls = [url for url in urls if url]
+        if any(fnmatch.fnmatch(url, pattern) for url in urls for pattern in patterns):
+            parser[section]["enabled"] = "0"
     buffer = StringIO()
     parser.write(buffer, space_around_delimiters=False)
     return buffer.getvalue()
 
 
-def _disable_existing_repos(context: Context) -> None:
+def _existing_index_urls(context: Context) -> set[str]:
+    """Collect the baseurl of every section of every .repo file already
+    present on the source filesystem."""
+    urls: set[str] = set()
     for entry in context.iter_src_dir(_REPOS_DIR):
         if entry.suffix != ".repo":
             continue
-        disabled = _disable_repo_content(entry.read_text(encoding="utf-8"))
+        parser = configparser.ConfigParser(interpolation=None)
+        parser.optionxform = str
+        parser.read_string(entry.read_text(encoding="utf-8"))
+        for section in parser.sections():
+            # noinspection SpellCheckingInspection
+            for key in ("baseurl", "mirrorlist"):
+                url = parser[section].get(key)
+                if url:
+                    urls.add(url)
+    return urls
+
+
+def _disable_existing_repos(context: Context, patterns: Sequence[str]) -> None:
+    for entry in context.iter_src_dir(_REPOS_DIR):
+        if entry.suffix != ".repo":
+            continue
+        disabled = _disable_repo_content(entry.read_text(encoding="utf-8"), patterns)
         context.write_file(f"{_REPOS_DIR}/{entry.name}", disabled.encode("utf-8"))
-        logger.info("disabled existing yum/dnf repository file %s", entry.name)
+        logger.info("copied existing yum/dnf repository file %s", entry.name)
 
 
 def _write_repo_file(context: Context, repository: YumRepository) -> None:
@@ -120,8 +152,28 @@ class YumModule(Module):
         return bool(yum.repositories) or bool(yum.keys) or any(existing_repo_files)
 
     def apply(self, context: Context) -> None:
-        _disable_existing_repos(context)
+        patterns = [
+            repository.index_url_to_replace
+            for repository in context.config.configurations.yum.repositories
+            if repository.index_url_to_replace is not None
+        ]
+        _disable_existing_repos(context, patterns)
+        existing_index_urls: set[str] | None = None
         for repository in context.config.configurations.yum.repositories:
+            if repository.index_url_to_replace is not None:
+                if existing_index_urls is None:
+                    existing_index_urls: set[str] = _existing_index_urls(context)
+                if not any(
+                    fnmatch.fnmatch(url, repository.index_url_to_replace)
+                    for url in existing_index_urls
+                ):
+                    logger.info(
+                        "skipped yum/dnf repository %s: index-url-to-replace %s "
+                        "matched no existing baseurl",
+                        repository.name,
+                        repository.index_url_to_replace,
+                    )
+                    continue
             _write_repo_file(context, repository)
         for key in context.config.configurations.yum.keys:
             _import_key(context, key)
diff --git a/library/id_configurator.py b/library/id_configurator.py
index bc93ff2..d3c7d73 100644
--- a/library/id_configurator.py
+++ b/library/id_configurator.py
@@ -167,6 +167,13 @@ options:
           description:
             type: str
             description: DEB822 "Description" field.
+          uri_to_replace:
+            type: str
+            description:
+              - fnmatch pattern checked against the URIs already present in
+                the machine's existing apt configuration. If set, this
+                source is only applied when the pattern matches at least
+                one of them; otherwise it is skipped entirely.
   apk:
     type: dict
     default: {}
@@ -185,6 +192,13 @@ options:
           key:
             type: str
             description: The signing key's name, if any (see NamedContent).
+          url_to_replace:
+            type: str
+            description:
+              - fnmatch pattern checked against the URLs already active in
+                the machine's existing /etc/apk/repositories. If set, this
+                repository is only applied when the pattern matches at
+                least one of them; otherwise it is skipped entirely.
   yum:
     type: dict
     default: {}
@@ -207,6 +221,13 @@ options:
           gpg_url:
             type: str
             description: The repository's gpgkey URL.
+          index_url_to_replace:
+            type: str
+            description:
+              - fnmatch pattern checked against the baseurl of every
+                section of every existing .repo file. If set, this
+                repository is only applied when the pattern matches at
+                least one of them; otherwise it is skipped entirely.
       keys:
         type: list
         elements: dict
@@ -491,6 +512,7 @@ ARGUMENT_SPEC = {
                     "check_date": {"type": "bool"},
                     "date_max_future": {"type": "str"},
                     "description": {"type": "str"},
+                    "uri_to_replace": {"type": "str"},
                 },
             },
         },
@@ -506,6 +528,7 @@ ARGUMENT_SPEC = {
                 "options": {
                     "url": {"type": "str", "required": True},
                     "key": {"type": "str"},
+                    "url_to_replace": {"type": "str"},
                 },
             },
         },
@@ -522,6 +545,7 @@ ARGUMENT_SPEC = {
                     "name": {"type": "str", "required": True},
                     "index_url": {"type": "str", "required": True},
                     "gpg_url": {"type": "str"},
+                    "index_url_to_replace": {"type": "str"},
                 },
             },
             "keys": {
diff --git a/tests/test_apk.py b/tests/test_apk.py
index 2a191cf..4adecc9 100644
--- a/tests/test_apk.py
+++ b/tests/test_apk.py
@@ -48,7 +48,9 @@ def test_is_relevant_true_with_apk_dir(tmp_path) -> None:
     assert ApkModule().is_relevant(context) is True
 
 
-def test_apply_disables_existing_and_adds_configured_repositories(tmp_path) -> None:
+def test_apply_copies_existing_repositories_unchanged_without_url_to_replace(
+    tmp_path,
+) -> None:
     src = tmp_path / "src"
     apk_dir = src / "etc/apk"
     apk_dir.mkdir(parents=True)
@@ -62,11 +64,13 @@ def test_apply_disables_existing_and_adds_configured_repositories(tmp_path) -> N
 
     ApkModule().apply(context)
 
+    # No configured repository declares a url_to_replace, so the existing
+    # lines are copied through unchanged, still active.
     content = (dst / "etc/apk/repositories").read_text(encoding="utf-8")
     assert content == (
         "# comment\n"
-        "# https://old-mirror.invalid/v3.18/main\n"
-        "# https://old-mirror.invalid/v3.18/community\n"
+        "https://old-mirror.invalid/v3.18/main\n"
+        "https://old-mirror.invalid/v3.18/community\n"
         "https://example.invalid/v3.19/main\n"
         "https://example.invalid/v3.19/community\n"
     )
@@ -75,6 +79,42 @@ def test_apply_disables_existing_and_adds_configured_repositories(tmp_path) -> N
     assert key_content == KEY_BYTES
 
 
+def test_apply_disables_only_existing_lines_matching_url_to_replace(tmp_path) -> None:
+    src = tmp_path / "src"
+    apk_dir = src / "etc/apk"
+    apk_dir.mkdir(parents=True)
+    (apk_dir / "repositories").write_text(
+        "https://old-mirror.invalid/v3.18/main\nhttps://old-mirror.invalid/v3.18/community\n",
+        encoding="utf-8",
+    )
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "apk": {
+                "repositories": [
+                    {
+                        "name": "main",
+                        "url": "https://example.invalid/v3.19/main",
+                        "url_to_replace": "https://old-mirror.invalid/v3.18/main",
+                    }
+                ]
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=Configuration.model_validate(data), dst=dst, src=src)
+
+    ApkModule().apply(context)
+
+    content = (dst / "etc/apk/repositories").read_text(encoding="utf-8")
+    assert content == (
+        "# https://old-mirror.invalid/v3.18/main\n"
+        "https://old-mirror.invalid/v3.18/community\n"
+        "https://example.invalid/v3.19/main\n"
+    )
+
+
 def test_apply_without_existing_repositories_file(tmp_path) -> None:
     src = tmp_path / "src"
     (src / "etc/apk").mkdir(parents=True)
@@ -90,6 +130,99 @@ def test_apply_without_existing_repositories_file(tmp_path) -> None:
     )
 
 
+def test_apply_skips_repository_when_url_to_replace_matches_nothing(tmp_path) -> None:
+    src = tmp_path / "src"
+    apk_dir = src / "etc/apk"
+    apk_dir.mkdir(parents=True)
+    (apk_dir / "repositories").write_text(
+        "https://old-mirror.invalid/v3.18/main\n", encoding="utf-8"
+    )
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "apk": {
+                "repositories": [
+                    {
+                        "name": "main",
+                        "url": "https://example.invalid/v3.19/main",
+                        "url_to_replace": "https://other.invalid/*",
+                    }
+                ]
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=Configuration.model_validate(data), dst=dst, src=src)
+
+    ApkModule().apply(context)
+
+    content = (dst / "etc/apk/repositories").read_text(encoding="utf-8")
+    assert "example.invalid" not in content
+
+
+def test_apply_writes_repository_when_url_to_replace_matches_existing(tmp_path) -> None:
+    src = tmp_path / "src"
+    apk_dir = src / "etc/apk"
+    apk_dir.mkdir(parents=True)
+    (apk_dir / "repositories").write_text(
+        "https://old-mirror.invalid/v3.18/main\n", encoding="utf-8"
+    )
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "apk": {
+                "repositories": [
+                    {
+                        "name": "main",
+                        "url": "https://example.invalid/v3.19/main",
+                        "url_to_replace": "https://old-mirror.invalid/*",
+                    }
+                ]
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=Configuration.model_validate(data), dst=dst, src=src)
+
+    ApkModule().apply(context)
+
+    content = (dst / "etc/apk/repositories").read_text(encoding="utf-8")
+    assert "https://example.invalid/v3.19/main" in content
+
+
+def test_apply_reuses_cached_existing_urls_and_handles_missing_file(tmp_path) -> None:
+    src = tmp_path / "src"
+    (src / "etc/apk").mkdir(parents=True)
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "apk": {
+                "repositories": [
+                    {
+                        "name": "main",
+                        "url": "https://example.invalid/v3.19/main",
+                        "url_to_replace": "https://old.invalid/*",
+                    },
+                    {
+                        "name": "community",
+                        "url": "https://example.invalid/v3.19/community",
+                        "url_to_replace": "https://old.invalid/*",
+                    },
+                ]
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=Configuration.model_validate(data), dst=dst, src=src)
+
+    ApkModule().apply(context)
+
+    assert not (dst / "etc/apk/repositories").exists()
+
+
 def test_apply_writes_nothing_when_no_repositories_at_all(tmp_path) -> None:
     empty_config = Configuration.model_validate(
         {"uuid": "12345678-1234-5678-1234-567812345678", "organisation": "InterDiode"}
diff --git a/tests/test_apt.py b/tests/test_apt.py
index 5accb33..4c9a348 100644
--- a/tests/test_apt.py
+++ b/tests/test_apt.py
@@ -70,7 +70,9 @@ def test_is_relevant_true_with_apt_dir(tmp_path) -> None:
     assert AptModule().is_relevant(context) is True
 
 
-def test_apply_disables_existing_sources_and_adds_configured_ones(tmp_path) -> None:
+def test_apply_copies_existing_sources_unchanged_without_uri_to_replace(
+    tmp_path,
+) -> None:
     src = tmp_path / "src"
     apt_dir = src / "etc/apt"
     sources_list_d = apt_dir / "sources.list.d"
@@ -97,24 +99,24 @@ def test_apply_disables_existing_sources_and_adds_configured_ones(tmp_path) -> N
 
     AptModule().apply(context)
 
-    disabled_top_level = (dst / "etc/apt/sources.list").read_text(encoding="utf-8")
+    # No configured source declares a uri_to_replace, so none of the existing
+    # entries are disabled: they are copied through unchanged.
+    top_level = (dst / "etc/apt/sources.list").read_text(encoding="utf-8")
     assert (
-        disabled_top_level
-        == "# already disabled\n# deb http://example.invalid/legacy stable main\n\n"
+        top_level
+        == "# already disabled\ndeb http://example.invalid/legacy stable main\n\n"
     )
 
-    disabled_list = (dst / "etc/apt/sources.list.d/extra.list").read_text(
-        encoding="utf-8"
-    )
-    assert disabled_list == "# deb http://example.invalid/extra stable main\n"
+    extra_list = (dst / "etc/apt/sources.list.d/extra.list").read_text(encoding="utf-8")
+    assert extra_list == "deb http://example.invalid/extra stable main\n"
 
-    disabled_sources = (dst / "etc/apt/sources.list.d/docker.sources").read_text(
+    docker_sources = (dst / "etc/apt/sources.list.d/docker.sources").read_text(
         encoding="utf-8"
     )
-    assert disabled_sources == (
-        "Types: deb\nURIs: http://example.invalid/docker\nSuites: stable\nEnabled: no"
+    assert docker_sources == (
+        "Types: deb\nURIs: http://example.invalid/docker\nSuites: stable\nEnabled: yes"
         "\n\n"
-        "Types: deb\nURIs: http://example.invalid/docker2\nSuites: stable\nEnabled: no\n"
+        "Types: deb\nURIs: http://example.invalid/docker2\nSuites: stable\n"
     )
 
     assert not (dst / "etc/apt/sources.list.d/ignored.txt").exists()
@@ -133,6 +135,55 @@ def test_apply_disables_existing_sources_and_adds_configured_ones(tmp_path) -> N
     )
 
 
+def test_apply_disables_only_existing_sources_matching_uri_to_replace(
+    tmp_path,
+) -> None:
+    src = tmp_path / "src"
+    apt_dir = src / "etc/apt"
+    sources_list_d = apt_dir / "sources.list.d"
+    sources_list_d.mkdir(parents=True)
+
+    (apt_dir / "sources.list").write_text(
+        "deb http://example.invalid/legacy stable main\n", encoding="utf-8"
+    )
+    (sources_list_d / "extra.list").write_text(
+        "deb http://example.invalid/extra stable main\n", encoding="utf-8"
+    )
+    (sources_list_d / "docker.sources").write_text(
+        "Types: deb\nURIs: http://example.invalid/extra\nSuites: stable\nEnabled: yes\n"
+        "\n"
+        "Types: deb\nURIs: http://example.invalid/docker2\nSuites: stable\n"
+        "\n\n",
+        encoding="utf-8",
+    )
+
+    dst = tmp_path / "dst"
+    context = Context(
+        config=make_config_with({"uri-to-replace": "http://example.invalid/extra"}),
+        dst=dst,
+        src=src,
+    )
+
+    AptModule().apply(context)
+
+    # The legacy source doesn't match the pattern, so it stays enabled...
+    top_level = (dst / "etc/apt/sources.list").read_text(encoding="utf-8")
+    assert top_level == "deb http://example.invalid/legacy stable main\n"
+
+    # ...but every entry whose URI matches is disabled, regardless of format.
+    extra_list = (dst / "etc/apt/sources.list.d/extra.list").read_text(encoding="utf-8")
+    assert extra_list == "# deb http://example.invalid/extra stable main\n"
+
+    docker_sources = (dst / "etc/apt/sources.list.d/docker.sources").read_text(
+        encoding="utf-8"
+    )
+    assert docker_sources == (
+        "Types: deb\nURIs: http://example.invalid/extra\nSuites: stable\nEnabled: no"
+        "\n\n"
+        "Types: deb\nURIs: http://example.invalid/docker2\nSuites: stable\n"
+    )
+
+
 def test_apply_without_existing_top_level_sources_list(tmp_path) -> None:
     src = tmp_path / "src"
     (src / "etc/apt").mkdir(parents=True)
@@ -200,6 +251,106 @@ def test_apply_without_signed_by_writes_no_field(tmp_path) -> None:
     )
 
 
+def test_apply_skips_source_when_uri_to_replace_matches_nothing(tmp_path) -> None:
+    src = tmp_path / "src"
+    apt_dir = src / "etc/apt"
+    apt_dir.mkdir(parents=True)
+    (apt_dir / "sources.list").write_text(
+        "deb http://example.invalid/legacy stable main\n", encoding="utf-8"
+    )
+
+    dst = tmp_path / "dst"
+    context = Context(
+        config=make_config_with({"uri-to-replace": "http://other.invalid/*"}),
+        dst=dst,
+        src=src,
+    )
+
+    AptModule().apply(context)
+
+    assert not (dst / "etc/apt/sources.list.d/interdiode.sources").exists()
+
+
+def test_apply_writes_source_when_uri_to_replace_matches_existing_uri(
+    tmp_path,
+) -> None:
+    src = tmp_path / "src"
+    apt_dir = src / "etc/apt"
+    apt_dir.mkdir(parents=True)
+    (apt_dir / "sources.list").write_text(
+        "deb http://legacy.invalid/debian stable main\n", encoding="utf-8"
+    )
+
+    dst = tmp_path / "dst"
+    context = Context(
+        config=make_config_with({"uri-to-replace": "http://legacy.invalid/*"}),
+        dst=dst,
+        src=src,
+    )
+
+    AptModule().apply(context)
+
+    assert (dst / "etc/apt/sources.list.d/interdiode.sources").exists()
+
+
+def test_apply_skips_source_when_uri_to_replace_set_without_src(tmp_path) -> None:
+    dst = tmp_path / "dst"
+    context = Context(
+        config=make_config_with({"uri-to-replace": "http://*"}), dst=dst, src=None
+    )
+
+    AptModule().apply(context)
+
+    assert not (dst / "etc/apt/sources.list.d/interdiode.sources").exists()
+
+
+def test_apply_matches_uri_to_replace_across_list_line_variants(tmp_path) -> None:
+    src = tmp_path / "src"
+    apt_dir = src / "etc/apt"
+    sources_list_d = apt_dir / "sources.list.d"
+    sources_list_d.mkdir(parents=True)
+
+    (apt_dir / "sources.list").write_text(
+        "# a comment\n"
+        "\n"
+        "garbage line here\n"
+        "deb [unterminated bracket with no closing marker\n"
+        "deb [arch=amd64 trusted=yes] http://example.invalid/extra stable main\n",
+        encoding="utf-8",
+    )
+    (sources_list_d / "ignored.txt").write_text("not a source file\n", encoding="utf-8")
+
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "apt": {
+                "sources": [
+                    {
+                        "name": "first",
+                        "uris": ["http://example.invalid/first"],
+                        "suites": ["stable"],
+                        "uri-to-replace": "http://example.invalid/extra",
+                    },
+                    {
+                        "name": "second",
+                        "uris": ["http://example.invalid/second"],
+                        "suites": ["stable"],
+                        "uri-to-replace": "http://example.invalid/extra",
+                    },
+                ],
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=Configuration.model_validate(data), dst=dst, src=src)
+
+    AptModule().apply(context)
+
+    assert (dst / "etc/apt/sources.list.d/first.sources").exists()
+    assert (dst / "etc/apt/sources.list.d/second.sources").exists()
+
+
 def test_apply_sanitizes_source_name_with_slash(tmp_path) -> None:
     dst = tmp_path / "dst"
     context = Context(
diff --git a/tests/test_yum.py b/tests/test_yum.py
index fa727a4..dd10c12 100644
--- a/tests/test_yum.py
+++ b/tests/test_yum.py
@@ -126,7 +126,9 @@ def test_apply_unsigned_repo_disables_gpgcheck(tmp_path) -> None:
     )
 
 
-def test_apply_disables_existing_repos_before_adding_configured_ones(tmp_path) -> None:
+def test_apply_copies_existing_repos_unchanged_without_index_url_to_replace(
+    tmp_path,
+) -> None:
     src = tmp_path / "src"
     repos_dir = src / "etc/yum.repos.d"
     repos_dir.mkdir(parents=True)
@@ -148,13 +150,13 @@ def test_apply_disables_existing_repos_before_adding_configured_ones(tmp_path) -
 
     YumModule().apply(context)
 
-    disabled_multi = (dst / "etc/yum.repos.d/multi.repo").read_text(encoding="utf-8")
-    assert "[old-one]" in disabled_multi
-    assert "[old-two]" in disabled_multi
+    # No configured repository declares an index_url_to_replace, so the
+    # existing sections are copied through unchanged, still enabled.
+    copied_multi = (dst / "etc/yum.repos.d/multi.repo").read_text(encoding="utf-8")
     parser = ConfigParser(interpolation=None)
-    parser.read_string(disabled_multi)
-    assert parser["old-one"]["enabled"] == "0"
-    assert parser["old-two"]["enabled"] == "0"
+    parser.read_string(copied_multi)
+    assert parser["old-one"]["enabled"] == "1"
+    assert parser["old-two"]["enabled"] == "1"
 
     assert not (dst / "etc/yum.repos.d/ignored.txt").exists()
 
@@ -165,6 +167,152 @@ def test_apply_disables_existing_repos_before_adding_configured_ones(tmp_path) -
     )
 
 
+def test_apply_disables_only_existing_sections_matching_index_url_to_replace(
+    tmp_path,
+) -> None:
+    src = tmp_path / "src"
+    repos_dir = src / "etc/yum.repos.d"
+    repos_dir.mkdir(parents=True)
+    (repos_dir / "multi.repo").write_text(
+        "[old-one]\nname=old-one\nbaseurl=https://old-one.invalid\nenabled=1\ngpgcheck=1\n"
+        "\n"
+        "[old-two]\nname=old-two\nbaseurl=https://old-two.invalid\nenabled=1\n",
+        encoding="utf-8",
+    )
+
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "yum": {
+                "repositories": [
+                    {
+                        "name": "unsigned",
+                        "index-url": "https://example.invalid/yum",
+                        "index-url-to-replace": "https://old-one.invalid",
+                    }
+                ]
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=make_config(data), dst=dst, src=src)
+
+    YumModule().apply(context)
+
+    disabled_multi = (dst / "etc/yum.repos.d/multi.repo").read_text(encoding="utf-8")
+    parser = ConfigParser(interpolation=None)
+    parser.read_string(disabled_multi)
+    assert parser["old-one"]["enabled"] == "0"
+    assert parser["old-two"]["enabled"] == "1"
+
+
+def test_apply_skips_repository_when_index_url_to_replace_matches_nothing(
+    tmp_path,
+) -> None:
+    src = tmp_path / "src"
+    repos_dir = src / "etc/yum.repos.d"
+    repos_dir.mkdir(parents=True)
+    (repos_dir / "old.repo").write_text(
+        "[old]\nname=old\nbaseurl=https://old.invalid\nenabled=1\n", encoding="utf-8"
+    )
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "yum": {
+                "repositories": [
+                    {
+                        "name": "unsigned",
+                        "index-url": "https://example.invalid/yum",
+                        "index-url-to-replace": "https://other.invalid/*",
+                    }
+                ]
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=make_config(data), dst=dst, src=src)
+
+    YumModule().apply(context)
+
+    assert not (dst / "etc/yum.repos.d/unsigned.repo").exists()
+
+
+def test_apply_writes_repository_when_index_url_to_replace_matches_existing(
+    tmp_path,
+) -> None:
+    src = tmp_path / "src"
+    repos_dir = src / "etc/yum.repos.d"
+    repos_dir.mkdir(parents=True)
+    (repos_dir / "old.repo").write_text(
+        "[old]\nname=old\nbaseurl=https://old.invalid/yum\nenabled=1\n",
+        encoding="utf-8",
+    )
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "yum": {
+                "repositories": [
+                    {
+                        "name": "unsigned",
+                        "index-url": "https://example.invalid/yum",
+                        "index-url-to-replace": "https://old.invalid/*",
+                    }
+                ]
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=make_config(data), dst=dst, src=src)
+
+    YumModule().apply(context)
+
+    assert (dst / "etc/yum.repos.d/unsigned.repo").exists()
+
+
+def test_apply_matches_index_url_to_replace_with_unrelated_files_and_multiple_repos(
+    tmp_path,
+) -> None:
+    src = tmp_path / "src"
+    repos_dir = src / "etc/yum.repos.d"
+    repos_dir.mkdir(parents=True)
+    (repos_dir / "old.repo").write_text(
+        "[old]\nname=old\nbaseurl=https://old.invalid/yum\nenabled=1\n",
+        encoding="utf-8",
+    )
+    (repos_dir / "ignored.txt").write_text("not a repo file\n", encoding="utf-8")
+
+    data = {
+        "uuid": "12345678-1234-5678-1234-567812345678",
+        "organisation": "InterDiode",
+        "configurations": {
+            "yum": {
+                "repositories": [
+                    {
+                        "name": "first",
+                        "index-url": "https://example.invalid/first",
+                        "index-url-to-replace": "https://old.invalid/*",
+                    },
+                    {
+                        "name": "second",
+                        "index-url": "https://example.invalid/second",
+                        "index-url-to-replace": "https://old.invalid/*",
+                    },
+                ]
+            }
+        },
+    }
+    dst = tmp_path / "dst"
+    context = Context(config=make_config(data), dst=dst, src=src)
+
+    YumModule().apply(context)
+
+    assert (dst / "etc/yum.repos.d/first.repo").exists()
+    assert (dst / "etc/yum.repos.d/second.repo").exists()
+
+
 def test_apply_sanitizes_repository_and_key_names_with_slash(
     tmp_path, monkeypatch
 ) -> None: