Welcome..
I don't think legacy exchangedn might come into picture here since we are not creating new mailboxes for these users.
We are just mount a blank EDB file so that mailbox GUID is intact.
But anyways for safer side you can take out the legacy exchangedn from existing mailboxes as suggested by MAS.
Thanks All, will let you know the result after the weekend.
That worked better than I thought, renaming the edb file was not enough as the log were a problem, I just moved all into a temp folder and it created a new database fine.
Thank you Saif I will be trying that this weekend.