DiRT's the way to go here - it doesn't care if the back end changes, the domain changes, if it's VM/UM or whatever.
Just install the new equipment into the domain using whatever server name you want (no need for it to be the same). Make sure the version of Unity you install is the same rev as the one you're moving. Do a backup of the existing system and a restore into the new system and you should be good to go.
Be sure to head out to www.CiscoUnityTools.com and grab the very latest versions of DiRT backup and restore before heading out...