It should be a matter of adding the new server to the same Exchange orginization and moving the mail boxes to the new server, and then once all the mail boxes have been moved over give it a few days then power down the old machine.
Buddy here, if you only have mailboxes then that might be fine. but if you have public folders that will not work. you will have to replicate the folders over. it was a nightmare on our 5.5 to 2003 migration and MS says it is the same path to go from 2003 to 2007. I recommend dumping the public folders to a pst file, then importing them back into your new server. you might have to reset up your security on the folders though. we didn’t go that route. when you migrate them to the new server it can take days even weeks for them to migrate and while they are migrating to the new server they are NOT accessible on the old OR the new server. None of them are accessible until ALL folders are done replicating. we let it go for 10 days and it still did not finish. We had about 140 folders. There also is NO way to tell how much is done or how much longer it is going to take. It is a horrible way to migrate and you think MS would have it figured out by now. I hear that when 2008 comes out their fix for this was to get rid of public folders all together. Not really a fix, just a way for them to get out of fixing it.
We finally scrapped it and went to Distributed File System. it is much better. Even MS recommends that you do NOT use public folders as a file system. unfortunately our users were doing just that. they kept common files for meetings and such. Bad idea. Distributed File System lets you setup a common file share point and then tell that to point to where you want the files to be. that way if a server crashes you restore the files to another server and change the pointer to go to the new server. the end users still point to the DFS pointer and their mappings do not have to be changed. Also in DFS you can setup replication so that it can replicate to two different shares. if the main share crashes the user gets pointed to the backup and they don’t see any problems at all. MS is supposed to do away with DFS too unfortunately. They are moving everyone to SharePoint server which I believe is another product you have to pay for. DFS came with the OS.