What I see most people on the Fediverse (who would normally just change their username if they could) do is to make a new account, then edit the profile info of the old account with a note that you've moved to the new account, with a link to it. That way your old submissions and comments are still around to help others, if anyone wants to contact you later they can, and you still get your changed username. That's what I do, but you can do whatever you want! :)