diff --git a/gclient_scm.py b/gclient_scm.py
index 3b1b59f7a..8acc4250c 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -698,15 +698,24 @@ class GitWrapper(SCMWrapper):
       # Switch over to the new upstream
       self._Run(['remote', 'set-url', self.remote, url], options)
       if mirror:
-        # Because we use Git alternatives, our existing repository is not
-        # self-contained. It's possible that new git alternative doesn't have
-        # all necessary objects that the current repository needs. Instead of
-        # blindly hoping that new alternative contains all necessary objects,
-        # keep the old alternative and just append a new one on top of it.
-        with open(os.path.join(
-            self.checkout_path, '.git', 'objects', 'info', 'alternates'),
-            'a') as fh:
-          fh.write("\n" + os.path.join(url, 'objects'))
+        if git_cache.Mirror.CacheDirToUrl(
+            current_url.rstrip('/')) == git_cache.Mirror.CacheDirToUrl(
+                url.rstrip('/')):
+          # Reset alternates when the cache dir is updated.
+          with open(
+              os.path.join(self.checkout_path, '.git', 'objects', 'info',
+                           'alternates'), 'w') as fh:
+            fh.write(os.path.join(url, 'objects'))
+        else:
+          # Because we use Git alternatives, our existing repository is not
+          # self-contained. It's possible that new git alternative doesn't have
+          # all necessary objects that the current repository needs. Instead of
+          # blindly hoping that new alternative contains all necessary objects,
+          # keep the old alternative and just append a new one on top of it.
+          with open(
+              os.path.join(self.checkout_path, '.git', 'objects', 'info',
+                           'alternates'), 'a') as fh:
+            fh.write("\n" + os.path.join(url, 'objects'))
       self._EnsureValidHeadObjectOrCheckout(revision, options, url)
       self._FetchAndReset(revision, file_list, options)