I can check to see if there's anything to assist in that with just OS X Mail Server usage, but I do know that Kerio includes a feature in its application to migrate Exchange to the Kerio mail server on OS X.
I do not know what kind of files you have on the exchange server. I have transferred mail from os x 10.3.4 to 10.4.1 and have a script for it (expects things like mbox files). I can give you that script (specify the version) and i can mail them to you on monday.
I had one server doing all the stuff. I had a problem after an upgrade from apple (to 10.3.9). Afterwards i move the mailserver to a macmini, but finally took the decision to add a xserve g5. But because harddrives are quite expensive, i moved the mailserver (all internet stuff actually) to the xserve and kept the powermac g5 for fileserver.
As i had problems getting my mail transfered, after some time a created the script to take care of it. Simple script but if you don't have, you have a lot of problems getting the mail to another server.
Sure, moving between Panther and Tiger isn't an issue. I've ripped out mail stores from Panther and dumped them into a fresh Tiger Server install and got that back up and running. What he's asking about is taking a Microsoft Exchange 2000 mail server and migrating it to Tiger. Panther and Tiger used fairly much the same background tools and formats, while Exchange does not.