IIRC, he's done it once or twice when he's really irritated with Gar.
And I suspect the reason they changed Gar's name to Beast Boy was because while Gar was in comic limbo, a game company began producing an RPG called "Changeling: The Dreaming". I think TPTB may have decided it was easier to go back to the old name than to get into a law suit over "Changeling".