Before migrating users, there is a bug in the process. The Profile service does not get updated properly until you apply Service Pack 1 and the June 2011 Cumulative Update. Within the June CU is this note:
Profile synchronization does not support domain migration in Microsoft SharePoint Server 2010.
This was a bug found internally at Microsoft which causes the Profile database to not be updated when a user is migrated between domains. If you’re unable to apply SP1, there is the post-April hotfix package KB2534417 that is also available.