How do I delete a mailbox and still keep the emails?

Question

I recently moved to another state and changed my ISP, so I need to delete an old mailbox that is no longer functional. I don't want to erase any of the email from that account, though, and am concerned that if I delete the account in my mail preferences I will lose those emails, although I have moved them from the inbox of the old account into other folders I created within the mail program. Any wisdom, anyone?
Thanks-
mg

Answer 1

Thank you for your suggestions. I've had a few bugs crop up in the process, though. My first step was to move all 659 "sent" emails from the sent box of the old account to the sent box of my current .mac account. I hit command "A" to select all of the sent emails in the sent box window, then selected "move to" from "message" in the menu bar and selected the .mac sent box as the destination. It seemed to take at least 15 minutes to complete the process, and although the sent box of the old account was empty, it appeared when I clicked on the .mac sent box that not quite all of the 659 emails had transferred, missing almost 300 sent emails.
I kept going though, and as suggested, quit Mail, and then made a copy of my user mail folder as suggested, dragging the folder to my desktop while holding down the option (alt) key. Then I reopened Mail, went to preferences, and deleted my old account. After that, I clicked through my remaining "In" mail boxes from the current accounts and the folders I had created in "On My Mac", and wierdly, it seemed every email had disappeared.
I didn't panic because I had made a copy to my desktop, so I quit Mail, and then reopened it, and it seemed everything was there, although still not all of the sent emails. Also when clicking through, Mail seemed to be functioning slowly, and the disappearing email thing happened again. I thought maybe Mail was getting confused between the mail folder in my user folder, and the copy on my desktop, so I moved the copy I had made to the trash, but didn't delete it just in case I needed it again. Then I quit Mail, went to disk utility, fixed permissions, shut down my computer, rebooted my internet cable modem and router, restarted my mac, and when I opened Mail, all my emails were there, including the emails I had moved to folders "On My Mac" from the old account. The old account was deleted, but the Sent box still seemed to be retrieving emails. When it finished, I checked the Sent box, and all of the emails I had transfered from the old account's Sent box were there. Whew.
It looks like Mail is working correctly, all the emails from my old account I wanted to keep are there in my "On My Mac" folders, and the old account is deleted. I'm not sure what happened when all the email in Mail initially disappeared right after I deleted my old account, but I'm sure there's an explanation. Copying my user mail folder before I deleted my old account did save me a panic attack, so I would highly recommend that suggestion I was given in case of problems or bugginess.
Thanks for your help!
pmg