I swear I vaguely remember it being in the EULA at one stage, but its either never been or has been recently removed. Which could throw into question as to whether its possible if someone wanted to get really arsey about it (but lets be honest, who would over a name).
Ever heard of key fields in a database (which have to be unique so you can assign data to it)?
That's exactly what your username is... And if you start messing around with those key data, chances are you mess up the whole database...
And as those before me pointed out: It's not like you haven't been warned...
You can change you driver's name as often as you want, just rename or create a profile and there you go...