So I wanted to change my "online ID" and found that impossible, so I just created a new account (knowing I would lose my online progress on the old account). I got that taken care of, but now every time I go to play online it tells me I have an account mismatch. I say ok, make a new guy, play with him, but it never saves anything. Everytime I go to play it tells me I have a mismatch and I have to start over.

What am I doing wrong?

You might need to delete the R6 file in your Saved Game Data directory.