Migrating Planning users from one domain to another and retain Planning permissions

This is more of a "technical, backend" question related to Hyperion Planning. 

A while back, my company acquired another, and then we had two domains in our Active Directory "Forest".  Hyperion Planning, via Shared Services, was set up to search for users in both domains - no problems, that works.  Planning now has users from both domains who have been assigned certain permissions in Planning. 

For purposes of this discussion, let's call the primary domain "original.com", and the acquired company "new.company.com". Now there is a project to migrate users from new.company.com to original.com.  In doing so, we would like to preserve each user's original Planning permissions as they get migrated from