diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -1074,6 +1074,11 @@
 )
 coreconfigitem(
     b'experimental',
+    b'sharesafe-auto-upgrade-shares',
+    default=False,
+)
+coreconfigitem(
+    b'experimental',
     b'single-head-per-branch',
     default=False,
 )
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -582,12 +582,19 @@
     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'
+            if ui.configbool(b'experimental', b'sharesafe-auto-upgrade-shares'):
+                storerequires = _readrequires(storevfs, False)
+                requirements = scmutil.upgrade_share_to_safe(
+                    hgvfs, requirements, storerequires
                 )
-            )
+                ui.warn(_(b'repository upgraded to use share-safe mode\n'))
+            else:
+                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/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -1597,6 +1597,22 @@
             fp.write(b"%s\n" % r)
 
 
+def upgrade_share_to_safe(hgvfs, current_requirements, store_requirements):
+    """Upgrades a share to use share-safe mechanism
+
+    Returns the set of new requirements for the repository
+    """
+    # after upgrade, store requires will be shared, so lets find
+    # the requirements which are not present in store and
+    # write them to share's .hg/requires
+    diffrequires = current_requirements - store_requirements
+    # add share-safe requirement as it will mark the share as share-safe
+    diffrequires.add(requirementsmod.SHARESAFE_REQUIREMENT)
+    writerequires(hgvfs, diffrequires)
+    current_requirements.add(requirementsmod.SHARESAFE_REQUIREMENT)
+    return current_requirements
+
+
 class filecachesubentry(object):
     def __init__(self, path, stat):
         self.path = path
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
@@ -479,8 +479,53 @@
   |
   o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
   
-  $ hg unshare -R ../nss-share
 
   $ hg log -GT "{node}: {desc}\n" -R ../ss-share
   abort: share source does not support exp-sharesafe requirement
   [255]
+
+
+Testing automatic upgrade of shares when config is set
+
+  $ hg debugupgraderepo -q --run --config format.exp-share-safe=True
+  upgrade will perform the following actions:
+  
+  requirements
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
+     added: exp-sharesafe
+  
+  processed revlogs:
+    - all-filelogs
+    - changelog
+    - manifest
+  
+  repository upgraded to share safe mode, existing shares will still work in old non-safe mode. Re-share existing shares to use them in safe mode New shares will be created in safe mode.
+  $ hg debugrequirements
+  dotencode
+  exp-sharesafe
+  fncache
+  generaldelta
+  revlogv1
+  sparserevlog
+  store
+  $ 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
+  
+  $ hg log -GT "{node}: {desc}\n" -R ../nss-share --config experimental.sharesafe-auto-upgrade-shares=true
+  repository upgraded to use share-safe mode
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+  
+
+Test that unshare works
+
+  $ hg unshare -R ../nss-share
+  $ hg log -GT "{node}: {desc}\n" -R ../nss-share
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+