No you cant change your username, but as long as you dont delete your cookies, you don't have to worry about forgeting it either, because your logged on until you log off. So if that was a concern problem solved.
Also when they generated a token for your eBay account, you gave auctiva premissions to access your listings, and updates your listing by placing your Store Window in the listing, you can change the design of you window in the store tab on auctiva.
Check out my listing window, and my store