Just remove the account and create a new one, but why not use IMAP and have the e-mails in gmail as well? That way you always have a backup in case something happens to the computer, you can read them from anywhere using the web interface or configure the account on a mobile phone/tablet and they will all be in sync about read/unread status etc.

I think gmail gives 15G of free storage these days... should be more than enough to keep using IMAP.

Additionally with IMAP you should be able to "upload" the folders to gmail, I never actually tried but I know it sees the existing directories (or rather, "labels") so there should be no reason why it wouldn't work.