diff -r 8a1a51d31e85 -r a07d5cb03a85 mercurial/upgrade_utils/engine.py
--- a/mercurial/upgrade_utils/engine.py	Mon May 03 12:22:06 2021 +0200
+++ b/mercurial/upgrade_utils/engine.py	Mon May 03 12:22:16 2021 +0200
@@ -80,12 +80,12 @@
 
     oldvfs = oldrl.opener
     newvfs = newrl.opener
-    oldindex = oldvfs.join(oldrl.indexfile)
-    newindex = newvfs.join(newrl.indexfile)
+    oldindex = oldvfs.join(oldrl._indexfile)
+    newindex = newvfs.join(newrl._indexfile)
     olddata = oldvfs.join(oldrl.datafile)
     newdata = newvfs.join(newrl.datafile)
 
-    with newvfs(newrl.indexfile, b'w'):
+    with newvfs(newrl._indexfile, b'w'):
         pass  # create all the directories
 
     util.copyfile(oldindex, newindex)