Would y'all count the following as fudging: Changing the tactical prowess of the bad guys and the response of reinforcements based on how the combat is going?
I definitely wouldn't call it "fudging," since that implies taking something 'solid' and treating it as fluid. If the die rolled a 15, it's usually taken to mean it's 15, not 'somewhere in the 13-17 range'. But what the monster choses to do with it's action is
already fluid - you were going to make the choice anyways, theoretically from any of the available options*, so any legal choice (which includes 'try something beyond the rules') is still within the parameters of the game.
* I'm assuming you mean "legal" moves, and not randomly adding new powers to a monster mid-fight.
Now, the reasoning behind your choice might be a factor:
If you're changing how the monsters behave because they have new information (including wrong information), that's just rp.
If you're changing monster behavior because you totally forgot they had an ability and now they start using it... that's fine.
If your changing monster behavior because the fight is getting boring and you want to mix it up... that's also fine. Players are free to do the same thing. (I often, as a player, use an ability in a less-than optimal way simply because I just got it and I want to use my new
toy spell.)
If you're changing monster behavior because you want a specific end result... that's getting into railroading territory, and this thread is controversial enough.
If you have multiple reasons, things get more complicated.