In previous Dragon Ball fighting games (such as the Budokai series), characters with multiple forms like Vegeta, Goku, and all your other favorite Saiyans could change forms mid-battle after performing certain supers. In Dragon Ball FighterZ, they're seperate fighters altogether, with different movesets.

Goku and Vegeta are the first to show off their Super Saiyan Blue forms from Dragon Ball Super. Both characters have new supers and special moves when played in their Blue forms, as you can see in the trailer below. All we need now is the younger Goku from the early sagas of Dragon Ball and fans can finally see their dream team of Goku, Goku, and Goku come to life.