I don't see the Pats defense (or offense or ST for that matter) trying to stay the same or trying to fundamentally change. I see it more as constant evolution.
For the academics on the board, check
this out for evolutionary theory that I think applies here as well. You kinda have to replace sexual reproduction with player acquisition...but what the hell, its the off-season.
A team can fall into the trap of spending all its energy and resources just to stay where they are. I think this includes the Colts now and will be the Chargers soon. The trap is that the competitive landscape is also changing and staying still quickly becomes falling behind when compared to all the other teams. It seems like Belichick really gets this. Player turnover and scheme adjustments are not only positives (when done on your terms), but they are necessary.
While I agree that Belichick is unlikely to change his long-standing defensive philosophy (3-4, 2-gap), I think it is foolish to believe he doesn't evolve the schemes (run, blitz, zone, man, etc.) from year to year.