diff --git a/tests/test-treemanifest-treeonly.t b/tests/test-treemanifest-treeonly.t
--- a/tests/test-treemanifest-treeonly.t
+++ b/tests/test-treemanifest-treeonly.t
@@ -419,6 +419,32 @@
    y |  1 +
    1 files changed, 1 insertions(+), 0 deletions(-)
   
-
-TODO
-# log -T "{manifest}" #TODO: edit templatekw.showmanifest
+Switch back to hybrid mode
+  $ cd ../client
+  $ cat >> .hg/hgrc <<EOF
+  > [treemanifest]
+  > treeonly=False
+  > EOF
+  $ hg pull
+  pulling from ssh://user@dummy/master
+  searching for changes
+  adding changesets
+  adding manifests
+  transaction abort!
+  rollback completed
+  abort: 00manifest.i@d9920715ba88: unknown parent!
+  [255]
+  $ hg backfillmanifestrevlog
+  adding changesets
+  adding manifests
+  adding file changes
+  added 0 changesets with 0 changes to 0 files
+  $ hg pull
+  pulling from ssh://user@dummy/master
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 0 changes to 0 files (+1 heads)
+  new changesets 4f84204095e0
+  (run 'hg heads .' to see heads, 'hg merge' to merge)
diff --git a/treemanifest/__init__.py b/treemanifest/__init__.py
--- a/treemanifest/__init__.py
+++ b/treemanifest/__init__.py
@@ -659,6 +659,36 @@
             recordmanifest(dpack, hpack, repo, mfrev1, mfrev2,
                            verify=opts.get('verify', False))
 
+@command('backfillmanifestrevlog', [
+    ], _('hg backfillmanifestrevlog'))
+def backfillmanifestrevlog(ui, repo, *args, **opts):
+    """Download any missing manifest revlog entries. This is useful when
+    transitioning back from a treeonly repo to a flat+tree hybrid repo."""
+    fallbackpath = getfallbackpath(repo)
+    with repo.connectionpool.get(fallbackpath) as conn:
+        remote = conn.peer
+
+        # _localrepo is needed for remotefilelog to work
+        if util.safehasattr(remote, '_callstream'):
+            remote._localrepo = repo
+
+        cl = repo.changelog
+        mfrevlog = repo.manifestlog._revlog
+
+        # We need to download any manifests the server has that we don't. We
+        # calculate that by saying we need all the public heads, and that we
+        # have some of them already. This might result in extra downloading but
+        # they become no-ops when attempting to be added to the revlog.
+        publicheads = repo.revs('heads(public())')
+        clnode = cl.node
+        heads = [clnode(r) for r in publicheads]
+        common = [clnode(r) for r in publicheads if
+                  cl.changelogrevision(r).manifest in mfrevlog.nodemap]
+        with repo.wlock(), repo.lock(), (
+             repo.transaction("backfillmanifest")) as tr:
+            cg = remote.getbundle('pull', common=common, heads=heads)
+            bundle2.applybundle(repo, cg, tr, 'pull', remote.url())
+
 @command('backfilltree', [
     ('l', 'limit', '10000000', _(''))
     ], _('hg backfilltree [OPTIONS]'))