We just completed the majority of such a migration. What we did is created the additional mailbox servers in VMware and then zmmailboxmove'ed the mailboxes. It was actually quite painless. We also put up a Proxy in front of the mailbox servers, so that as a mailbox was moved from one server to another, the user's server address stayed the same. The only issues we noticed were:
1.) If you have a BES, it will not work through the proxy at this point.
2.) If your users use IM, then they will loose their IM list after the move and will need to re-add all buddies.
3.) IM Clients will need to be re-configured to point directly to the mailbox server they are on. (Hoping that this will be fixed in the future and use the proxy)
4.) Ran into some caching issues when moving mailboxes:
Zmmailboxmove Results in Corrupted Mailboxes
The last part we have left is redirecting LDAP and Logger and promoting one of our mailbox servers to master LDAP server. This should be a task of following
Promoting Replica to LDAP Master and deleting the old server from LDAP once every account is moved.
Cheers,
Dusty