diff --git a/mercurial/upgrade.py b/mercurial/upgrade.py
--- a/mercurial/upgrade.py
+++ b/mercurial/upgrade.py
@@ -87,6 +87,7 @@
     up_actions = upgrade_actions.determine_upgrade_actions(
         repo, format_upgrades, optimizations, repo.requirements, newreqs
     )
+    removed_actions = upgrade_actions.find_format_downgrades(repo)
 
     removedreqs = repo.requirements - newreqs
     addedreqs = newreqs - repo.requirements
@@ -108,6 +109,7 @@
         newreqs,
         repo.requirements,
         up_actions,
+        removed_actions,
         revlogs,
     )
 
@@ -226,20 +228,4 @@
                     )
                 )
 
-            if upgrade_actions.sharesafe.name in addedreqs:
-                ui.warn(
-                    _(
-                        b'repository upgraded to share safe mode, existing'
-                        b' shares will still work in old non-safe mode. '
-                        b'Re-share existing shares to use them in safe mode'
-                        b' New shares will be created in safe mode.\n'
-                    )
-                )
-            if upgrade_actions.sharesafe.name in removedreqs:
-                ui.warn(
-                    _(
-                        b'repository downgraded to not use share safe mode, '
-                        b'existing shares will not work and needs to'
-                        b' be reshared.\n'
-                    )
-                )
+            upgrade_op.print_post_op_messages()
diff --git a/mercurial/upgrade_utils/actions.py b/mercurial/upgrade_utils/actions.py
--- a/mercurial/upgrade_utils/actions.py
+++ b/mercurial/upgrade_utils/actions.py
@@ -57,6 +57,14 @@
     upgrademessage
        Message intended for humans explaining what an upgrade addressing this
        issue will do. Should be worded in the future tense.
+
+    postupgrademessage
+       Message intended for humans which will be shown post an upgrade
+       operation when the improvement will be added
+
+    postdowngrademessage
+       Message intended for humans which will be shown post an upgrade
+       operation in which this improvement was removed
     """
 
     def __init__(self, name, type, description, upgrademessage):
@@ -64,6 +72,8 @@
         self.type = type
         self.description = description
         self.upgrademessage = upgrademessage
+        self.postupgrademessage = None
+        self.postdowngrademessage = None
 
     def __eq__(self, other):
         if not isinstance(other, improvement):
@@ -109,6 +119,14 @@
     # value of current Mercurial default for new repository
     default = None
 
+    # Message intended for humans which will be shown post an upgrade
+    # operation when the improvement will be added
+    postupgrademessage = None
+
+    # Message intended for humans which will be shown post an upgrade
+    # operation in which this improvement was removed
+    postdowngrademessage = None
+
     def __init__(self):
         raise NotImplementedError()
 
@@ -235,6 +253,19 @@
         b'shares of this repository share its requirements and configs.'
     )
 
+    postdowngrademessage = _(
+        b'repository downgraded to not use share safe mode, '
+        b'existing shares will not work and needs to'
+        b' be reshared.'
+    )
+
+    postupgrademessage = _(
+        b'repository upgraded to share safe mode, existing'
+        b' shares will still work in old non-safe mode. '
+        b'Re-share existing shares to use them in safe mode'
+        b' New shares will be created in safe mode.'
+    )
+
 
 @registerformatvariant
 class sparserevlog(requirementformatvariant):
@@ -585,6 +616,7 @@
         new_requirements,
         current_requirements,
         upgrade_actions,
+        removed_actions,
         revlogs_to_process,
     ):
         self.ui = ui
@@ -593,6 +625,7 @@
         # list of upgrade actions the operation will perform
         self.upgrade_actions = upgrade_actions
         self._upgrade_actions_names = set([a.name for a in upgrade_actions])
+        self.removed_actions = removed_actions
         self.revlogs_to_process = revlogs_to_process
         # requirements which will be added by the operation
         self._added_requirements = (
@@ -679,6 +712,15 @@
         """ Check whether the upgrade operation will perform this action """
         return name in self._upgrade_actions_names
 
+    def print_post_op_messages(self):
+        """ print post upgrade operation warning messages """
+        for a in self.upgrade_actions:
+            if a.postupgrademessage is not None:
+                self.ui.warn(b'%s\n' % a.postupgrademessage)
+        for a in self.removed_actions:
+            if a.postdowngrademessage is not None:
+                self.ui.warn(b'%s\n' % a.postdowngrademessage)
+
 
 ###  Code checking if a repository can got through the upgrade process at all. #