I just switched over to IMAP with Gmail and Apple Mail so messages I read on my iPhone show up as read in Mail, and vice versa.
No, you don't have to delete your account. Under Mail > Preferences > Accounts > Advanced, there's an option to disable your POP account. Don't worry, it just means the account, along with the messages, will be 'hidden' and won't show up in the main window unless you re-enable the account. Nothing will be deleted. If you already put your IMAP settings in, you can enable/disable that account the same way. Right now, I have my POP disabled and IMAP enabled. There's just one thing you gotta watch out for: If you decide to re-enable your POP account, every message you've received since you disabled it will start downloading.