I don't know, pro athletes seem to tweak their mechanics all the time, especially pitchers and hitters in baseball. Sure, old habits are hard to break, but these aren't weekend warriors we're talking about. They're highly paid professionals who can put their entire focus into something as seemingly mundane as the angle of your arm in a throwing motion.
Sure, some of the pros are better at adjusting mechanics than others, but everything we know about Fitz says that he's not just smart. He's a humble guy with a good deal of humility who had to work against all kinds of lowered expectations to get where he is today. I don't think his ego will be getting in the way of adjusting his mechanics and improving his game.