I can tell you the process we followed. We built the Exchange 2007 server and installed Exchange 2007 on it. A router connection was created during installation to communicate between the exchange servers. Then I just moved all mailboxes to the new server in nightly scheduled moves of about 500 each night. Make sure you have set limits on mail stores before the moves and everyone is aware of what the limits are. If you have public folders make sure you create a public folder store on the exchange 2007 server, it is not created by default. We have a separate public folder server for our Exchange 2007 organization. Replicate all folders to the exchange 2007 public folder store. This will allow free/busy search functionality between the exchange servers during the transition. Our exchange mailbox servers are CCR clusters and we built two CAS/hub transport servers separate from the mailbox servers. Good luck to you!