Official scorers work for MLB, not the home teams. And yes, he can change it anytime up to 24 hours after the end of the game. Teams can also appeal and have rulings reviewed by the League (or at least they could as of a few years ago).
So if it stands as only one hit for the angels the rest of the game, Marwin will have broken up a no hitter/perfect game and have lost a no hitter.