Background on the situation:
Company A purchases Company B. Company A is on Exchange 2010 and creates Mail Contact's for all users in Company B.
Users and Computers in Company B are all migrated to Company A using ADMT. Mailboxes were created for those users. Now there are two entries in the GAL for each user, one that is an old Mail Contact with external address, one that is a new Exchange User Mailbox.
Is there a way we can copy attribute(s) out of the old Mail Contacts, delete Mail Contacts, and enter legacy attribute(s) into the Mailboxes so that autocomplete continues to work for the rest of the Exchange Org users?