Will a Username change affect my sales?
I think that really depends to what extend your branding depends on it. If you have a characteristic avatar, and that is what people know you by, your name might not matter that much. It also depends on how large the change would be, I suppose.
I’m not sure if there are any statistics on the number of ‘return buyers’, for whom it might actually matter, but depending on the market you’re targetting that should also be taken into consideration.