Reb Wolf makes an excellent point. You CAN change your existing username. Go to:
https://www.theyeshivaworld.com/coffeeroom/users/ctlawyer/edit
Add a “Nickname”. Then switch your Display Name to the Nickname you just chose. Then click Update Profile on the bottom of the screen.
One thing to keep in mind is that doing this will also change your display name on all your old posts.