Well, once it's predetermined that's it. I don't think it's legal in real life to change something that's been predetermined. I'm pretty certain that in MLB when a manager makes a predetermination, he is not allowed to vary from it.
Hold on, I was wrong. The rulebook states if a pitcher is throwing a no hitter thru the 4th inning and has tossed less than 60 pitches, the predetermination can be reversed. But the manager has the final say, because there's no penalty for over-managing.