sidHistory & Portal Authentication (During Migration)

This is a follow up to this thread

http://social.technet.microsoft.com/Forums/en-US/058c8521-780b-4526-8301-c4bfbb86cc22/sidhistory-and-fim-portal-authentication-during-domain-migration?forum=ilm2

During a domain migration how can the Portal be utilized to accommodate both Migrated Users & Pre-Migrated Users?? I would not like to duplicate the whole user base by pulling both old domain accounts and new domain accounts.

Even though SharePoint can accommodate sidHistory, it seems the FIM web services cannot. It complains that it can't find the migrated "Domain\User" with {newSid}.