We've enabled our Governance customers to migrate users that do not own content currently on retention or legal hold to new EIDs. When those users are moved, any content they own that is not part of an active governance policy will move with them to the new EID. The rest of their content will remain in where it is - and if it is part of a governance policy will remain under that policy.
We are currently working to expand this capability to users who own content under retention or legal hold by letting admins define new owners for that content before the users gets moved to the new EID.