dude... you got ripped off... wow... $830?
anyway... i havent seen anyone flashing the Thunderbolt for another network like MetroPCS... I'm sure it can be done, but is not by rooting it... its my reprograming the accual carrier settings by entering in the dialer codes (##code#) and probably you need to get the MSL code to access this area.
Do a search or go to you local flea market, im sure there someone knows a guy that knows someone that does it

There is an underground for unlocking phones, especially in other countries so maybe if he waits, he may meet a guy who knows a guy too. But like someone else pointed out, this phone doesn't use simm cards so unlocking probably is not an option.
Oh, and that guy who sold you the phone? In his profile he lists way too many accomplishments but all he says is that they are in "corporate and freelance situations." That can be just about anything. His education is only a bachelors in Business Management. That doesn't mean you area tech geek, but it probably gets you an entry job in the mail room. LOL!!!
He ripped you off.