I just run "update sysobjects set loginame = NULL".
Other items to check ...
- Is the same login (and same suid) defined as the 'dbo' in the source and target dataservers?
- How do you address suid mismatches between syslogins vs sysusers/sysalternates, and role id mismatches between syssrvroles vs sysusers/sysroles? suid/role-id mismatches can wreak havoc with accessibility issues (including permissions)