Couple things.

First, you have to have the servers digitally networked to use the global subscriber migration tool.

http://www.ciscounitytools.com/App_GSM4x.htm

watch the video.

Basically, when you are in this view, you can move Unity Subscribers from one Unity server to another. (version does not matter) It will move their recorded name, personal greeting, and pin. (see the notes about the streaming folder being setup also)

I would not do this in bulk. Go slow. Take groups of 10-20 at a time. Seems to work best that way.

Once they are moved, migrate the subscribers to a new Exchange server.

You can test everything live with a couple users. Just plan on your integrations with CallManager or phone switch.

hope this helps.