If your email is an imap system, it's trivial -- you just point mail to the server and all of it will come in. If your folders are locally stored, it's not as trivial, but again if you have access to an imap account, you can just move or copy all your folders into that imap account, and then bring them into mail direct from the server. If you don't have imap, you'll have to export your email somehow, or use one of the tools you can find by searching on this board, which as I recall cost a few dollars.