In 1.9version, we will be getting this feature "change username ". Now it depends on nopCommerce Team whether they add this feature on this website or not.
Brian I would use https://www.nopcommerce.com/contacts.aspx to contact Andrei to get your username changed.
I would just write something custom to edit the usernames, upgrading is never really going to be option for you as you have such a customised version of nopcommerce (i.e multistore).