don't quote me on this ....
i dont think you need to change the usercode as i dont think the security goes that far (check the first xx of the user_code against the username).....
the user code is used to on the users computer it is put into a cookie and when the user comes to a page on the forum it gets the usercode from the users machine and looks for the user_code that matchs and then gets the relvent details.
if you changed the username the user_code should still work but the user wouldn't know his username has been changed untill he sees it on the forum... . as he wouldn't of logged in......
so no you dont have to change it....
any probbs ..reply back....