Oh, in Dragonall, EVERY villian got beat by Goku and became good.

Wel;l, Tien becoming good was actually a well executed one, with Master Roshi making Tien question who he was and who he wanted to be.

And no, Yamcha was never really a bad guy. He just started following the good guys in the hopes of getting the wish for himself and fell in love with Bulma along the way.