Sorry does E2k7 have to be the partner server to 5.0?
If not what features do we lose if any.
I did search for docs but did not find any.
Yeah, that will get yeah everytime. I do find that moving GSM is small groups of "20's" seem to work better than hundreds at a time. Also to wait after it's done before deleting or changing subscriber information. Just in case the database is still syncing up.
it's a two stage event. I would upgrade to Unity 5.x first and integrate to Exchange 2003.
Install your Exchange 2007 server
If you do not have a spare server to use for migration, you will have to move everyone to the new message store at the same time. (meaning every subscriber on Unity that uses 2003, be migrated to 2007 at the sametime) You will then have to re-run message store wizard and rehome from 2003 to 2007.
If you want to do this in phases, you need (2) unity servers. Unity 4.2 to Exchange 2003 and Unity 5.x to Exchange 2007. Digital Network the Unity servers and you can then migrate Exchange users at your pleasure without affecting Voicemail.