I can somewhat understand what you are seeking. Over on the Bionicle forum, BZPower, I originally joined back in 2004, but when I decided to resume activity last year after being absent for a few years, I decided I wanted to have a fresh start. They don't allow us to create second accounts; we're stuck with the first one we registered with. But they do allow us to change our usernames, but it comes with a waiting period of 90 days before you can change it again (and when you log in, you have to use the username you originally registered with instead of the one you currently have).
My advice to you for this site: Since you say you're leaving soon anyway, go ahead and leave, and if you decide to come back a few years from now, maybe create a new account with the new name you really want. That's pretty much all I can say about this.