I really dislike changes that invalidate decades of gameplay, though. If a monster that was always susceptible to charm person suddenly isn't, what if your campaign had seen major events played out where using charm person on that monster was a major plot point? If you aren't going to restart your game because of a new ruleset or edition- and I'll die on the hill that an edition change shouldn't force a reset- then edition changes should be made with that sort of situation in mind and changes to monsters should be made judiciously. You can update mechanics and change stat blocks in ways that don't run that risk.