It does appear to be a lock of some kind, have you tried restarting the server and testing it again? Perhaps migrate a new copy of the domain over, restart the server, then start SmarterMail to see if it successfully converts. Lastly; what are you using to migrate the data over? I've used rsync to move most migrated domains and haven't run into these issues.
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com