diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -557,6 +557,11 @@
     # NOTE: presence of SHARESAFE_REQUIREMENT imply that store requirement
     # is present. We never write SHARESAFE_REQUIREMENT for a repo if store
     # is not present, refer checkrequirementscompat() for that
+    #
+    # However, if SHARESAFE_REQUIREMENT is not present, it means that the
+    # repository was shared the old way. We check the share source .hg/requires
+    # for SHARESAFE_REQUIREMENT to detect whether the current repository needs
+    # to be reshared
     if requirementsmod.SHARESAFE_REQUIREMENT in requirements:
 
         if (
@@ -575,6 +580,15 @@
             storevfs = vfsmod.vfs(hgvfs.join(b'store'))
 
         requirements |= _readrequires(storevfs, False)
+    elif shared:
+        sourcerequires = _readrequires(sharedvfs, False)
+        if requirementsmod.SHARESAFE_REQUIREMENT in sourcerequires:
+            ui.warn(
+                _(
+                    b'warning: source repository supports share-safe functionality.'
+                    b' Reshare to upgrade.\n'
+                )
+            )
 
     # The .hg/hgrc file may load extensions or contain config options
     # that influence repository construction. Attempt to load it and
diff --git a/tests/test-share-safe.t b/tests/test-share-safe.t
--- a/tests/test-share-safe.t
+++ b/tests/test-share-safe.t
@@ -373,6 +373,7 @@
 Make sure existing shares still works
 
   $ hg log -GT "{node}: {desc}\n" -R ../nss-share
+  warning: source repository supports share-safe functionality. Reshare to upgrade.
   @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
   |
   o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo