I'm wondering if there is a migration strategy from VM only to UM, I'm being told that if you have Unity 4.1 you can, I was alwyas under the impression that once you have installed VM only that you would have to rebuild the server and install it as UM
A rebuild is for sure. You would probably go about it like this:
Spare drives for rollback.
Reload Unity server into UM domain (where Exchange server resides for email)
Load Unity 4.1 back
*** If Subscribers Alias's are different from VM and UM, you will need to create an Alias lookup CSV file. Basically this means if John Doe is jdoe on Unity VM, and in UM Exchange his alias is john.doe, you will need to create the Alias CSV reference file for DIRT to match the accounts up or Unity will create new AD and Exchange accounts for everyone!
If the Alias are the same, then it will be really easy for the DIRT restore to take place.
As for the messages, DIRT will backup the messages, however, if there a ton, it will be a long process. DIRT basically uses ExMerge to move to voicemails out to PST and then back to the new Exchange store with Exmerge. (matching alias to alias for importing.)
Hope this helps.