If your domain name is set up properly, then yes, you'll be able to transition without recreating the profiles. You want your autodiscover record (autodiscover.domainname.com) to be what updates the location. With autodiscover, Outlook pulls in the correct settings. This is what allows hosted servers to change server names all the time and do farm migrations without the end users knowing.
You'll need Mailflow, Split Domain, the new MX Records added as lower priority than your current MX records (so later you can swap them) and then change where your autodiscover record is pointing. Make sure you have all of your firewall policies in place to allow autodiscover and activesync to work properly into the exchange server (set up a test domain and test the crap out of it) and use the Microsoft Exchange Autodiscover and ActiveSync test tools on the web to test for proper configuration.
Once the test environment works for the flowing of connectivty through the firewall, then you know that the transition should work just fine.
As for the additional mailboxes, rather than configure them for multiple mailboxes, you can just grant their main account Full Access permissions to the secondary mailboxes and they can pull them up right in outlook as a secondary mailbox. Will look and feel the same, but the permissions are based on their main user account. Easier when someone gets their access restricted from such a situation because then you just revoke their permissions as opposed to having to get on to their system to remove the additional account.