Being in a bandwidth-impoverished economy, and not wishing to re-download about 10GB of mail, I made some scripts that let you set up a ZD2 account on a server, and then rsync the content back on top of your ZD1 account on your machine, so as not to have to transfer messages twice.

This will require manual adjustment for anyone else wanting to use it, but the basic outline is available at Zimbra Desktop 2 Storage Migration - Zimbra :: Wiki

Also contains some interesting discoveries about ZD2 storage (messages over about 140kb are gzipped, saving you storage space, and so on)