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This type of move would need in-depth planning and possibly hire consulting services to aid in this transition.
You could try this as a test ( on your test servers \ domains )....
Steps to manually move a user(s) from Domian1 to Domain2 :
1. Backup the user's mail database -
if necessary unencrypt with the user's ID file or change the Readers lists to make it readable by anyone with Manager access.
2. Print the user's Person Record for reference - select the document in the "People and Groups" tab of the admin client and print.
It will print all the pages.
3. Delete the user from Domain1 - use the "Delete Person" button in the admin client.
To reverse this you can register the user again into Domain1, make sure to remove the user's name from the Deny Access or Terminated Group.
4. Register a new user with the same name in Domain2.
Copy the user's new mail server in Domain2 - overwritting the new mail file just created by the Registration process.
Reference technotes
1086417,  How to move/recertify mass users to a new domain using AdminP ()
1106178,  Moving Notes users to a different domain and server using AdminP ,
1091738, Merged two domains to one, old mail reply to wrong domain Document
Feedback response number TOCL9ZJLJ8 created by ~Emile Chuhipiburoopsi on 08/19/2015