diff -r a07d5cb03a85 -r 396442cd7e6a mercurial/upgrade_utils/engine.py
--- a/mercurial/upgrade_utils/engine.py	Mon May 03 12:22:16 2021 +0200
+++ b/mercurial/upgrade_utils/engine.py	Mon May 03 12:22:26 2021 +0200
@@ -82,14 +82,14 @@
     newvfs = newrl.opener
     oldindex = oldvfs.join(oldrl._indexfile)
     newindex = newvfs.join(newrl._indexfile)
-    olddata = oldvfs.join(oldrl.datafile)
-    newdata = newvfs.join(newrl.datafile)
+    olddata = oldvfs.join(oldrl._datafile)
+    newdata = newvfs.join(newrl._datafile)
 
     with newvfs(newrl._indexfile, b'w'):
         pass  # create all the directories
 
     util.copyfile(oldindex, newindex)
-    copydata = oldrl.opener.exists(oldrl.datafile)
+    copydata = oldrl.opener.exists(oldrl._datafile)
     if copydata:
         util.copyfile(olddata, newdata)