Revit stores your username in the Options dialog, Application Menu (the Big R)> Options. Usually this is the same as the username you use to log into your computer. When you enter a different username Revit stores this in the Revit.ini. This stored username is persistent from this point forward. You'll have to manually change it if necessary. Keep in mind that there is a master Revit.ini and a user specific Revit.ini. You can edit them to make sure no username is specified and this will restore the original behavior, using the login username instead.
That's not the gist of the post though. When Worksets are enabled your username takes on greater significance. There should never be any identical usernames working on a workset project. If there are two Steve's then the first Steve that synchronizes with the central file wins. The other Steve sits in abject misery, quietly imagining how the first Steve will suffer. Don't be the second Steve.
Still not the point of the post...
You can change your username before you open a local file but not after. Once Revit identifies you with a file, that files is yours. You can't be someone else and work on that file. You can't change your username until you close that file. Then you can create a new local file after changing the username.
You can however change your username anytime you want when you work in a central file. It isn't a great idea to actually work this way, switching usernames as you go. It is however a way to clear out users that have not relinquished worksets properly though. When nobody else is working on the project you can open a central file and pretend to be the ill mannered users that haven't relinquished elements properly. It's another reason you might consider working in a central file, even though it is generally frowned upon.
This is not something you should do casually. If you do this to someone who hasn't saved changes yet, just wasn't ready to do so, you will prevent them from doing so. This won't make you popular at the office. It is an awesome way to resolve little matters when people are all out of the model. In a perfect world you'd never need to do it. Sadly I don't live in a perfect world. If you do, invite me over sometime?