I've done a profile folder rename once or twice, but forget the exact details. I think it was as simple as:

Like Bitt said, make a new account and log in.
Log out and back in as admin.
MOVE the contents of the old profile folder to the new folder, overwriting whatever is there.

This moves the old user's Application Data (Firefox bookmarks, Outlook PSTs), user portion of the registry, My Docs, Desktop, Start Menu, etc. I think it's as simple as that, but I'd have to check my notes at work.