First, I never said head coaches can't or won't change their systems. I said great head coaches/GMs find great players who fit their system and only change the system to match the player if they get a player they know is a potential HOFer or at least strongly believe it. Otherwise, it isn't worth it. Belichick did change his defense quite a bit with both Talib and Revis. Before those two, the Pats were a zone base and went to a man base. But didn't totally defense. Just made major changes.
Second, to my last point. Belichick is constantly tweaking both his offense and defense to fit the talent he has. Again, he isn't making wholesale changes, but he is doing things to put his players in the best position to succeed. The Pats have moved back to a cover 2 zone this season because he doesn't have two outside CBs he can trust to cover the outside receivers one on one.
Third, I never said Belichick would ever try to force fit a QB into his system. At least not long term. Again, that is why I said that he drafted Jones. Jones' playing style is perfect for the Pats' offensive system. He is very Brady like in his style which is what attracted Belichick to him since this offense needs a smart pocket passer who can go through his progressions quickly, throws with anticipation because this is a timing offense, can read the defense and knows the route adjustments, and throws into tight windows. He was lucky enough to have a guy who fits that bill drop on his lap this draft. Word was they didn't want Fields. Why? Most likely because although he has All World potential, he doesn't possess most of the qualities I just listed. Or at least they are his strengths.