Account names and player names are different. We are going to add a system that will allow you to add players, delete them, change their names, etc. This will reflect in the game, but will not affect your stardock account name.
EDIT: It looks like we have a job that runs every once in a while and updates the nicknames in Demigod. In other words, if you change your account name, wait a little while and your in-game name will change too.
Bara