So I looked around Smashwords and could not find a good answer to your question. What I did find is that for my published books payee information and author information are completely separate entries that can be edited independently.
"Username" is only for your web url as far as I can tell.
If you go to
https://www.smashwords.com/account/edit
It appears that what you put in "Firstname" and "Lastname" there is the pen name and can be changed at any time.
I would wait to hear back from Smash though, because I can not be sure as I do not use a pen name.