There are different paths you could take and third party softwares out there to help you make this transition, however if you don’t want to spend the money the simplest thing to do is install the exchange 2007 servers and simply move the mailboxes to them using the exchange tools. Exchange 2007 allows you to schedule bulk move operations on more than one server. When we did our moves I used both mailbox servers and both CAS/HUB servers to schedule and accomplish the moves. I scheduled them at night and on the weekends and had no problems at all. I moved 10,000 plus mailboxes over a period of about a month. We took our time and notified users which night their mailboxes would be moved.
Here is an excellent article to help you as well.