Re: Migrating DE 7.1.3 clients from one ePO to another ePO in different domain
I did a successful migration of about 500 encrypted systems from 1 ePO server (4.6.8) in one forest to another ePO server (5.3.2) in another forest. The other hitch was that the systems and users themselves were also being migrated to a different forest. I think your scenario is actually a little bit easier.
Long story short - Register the AD domains as Registered Servers. Enable System transfer via Web-API. As long as the ePO server can find the already assigned users in Active Directory, it will "ship" them back up to ePO and assign them to the leafNodes.
Take a look at the System Transfer feature that was introduced with McAfee Drive Encryption 7.1.3 and up.