What is the difference between "oh crap, that roll was max damage, I'm...just going to pretend the dice were average" and "oh crap, that roll was max damage, I'm...just going to pretend that the monster has a -5 damage modifier instead of whatever positive modifier it has." Or even "I'm going to pretend that the dice were d4s rather than d10s." Modifying a monster mid-combat, without diegetic justification (and thus a justification that is reasonably learn-able by the players, even if they coincidentally end up not learning of it) has identical effects to replacing its rolls with different rolls. I think the only possible exception is specifically to do with crits, but even then there are ways that modifications end up doing the same thing. It'll just be a more roundabout way.