Hi,
This is really more of an MS question.
But I take it you are using Unity VM only for exchange? If it is a separate AD then the only info in that AD is used for Unity so you dont really need to keep any of it.
If it's in a separate AD then why not just configure your exchange 2007 for UM in parallel.
In CallManager you can configure a separate voicemail profile for Exch UM and assign it to certain users.
Have you piloted this yet? There are some limitations with UM for Exch 2007.
You can export subscriber information from SQL database on unity, and use it with powershell script for exch 2007 to actually get the users across.
I'd say when people move, they are going to have to re-record greetings, and lose their old voicemail etc.
Cheers,
Tim