That's not really fudging, it's winging it.
There's very little difference between not picking an AC, and then just deciding at some point in the fight that it's 16 vs. having the AC be 18, but never let the players know, and just decide that their 16 hits anyway so now the monster's AC is 16. Both are fudging. Though it seems like every time a Fudging thread pops up, eventually people start to pick apart the definition of fudging, and we find out everyone has a different interpretation of it.
I'll just say that for me, Fudging is any time a DM changes an outcome based on their own desires, and not the pre-established mechanics of the game or encounter. Turning a crit into a normal hit because you don't want to kill a PC is the DM's desire, not the outcome of the dice. Increasing the HP of a monster that started at 100 to 200 because you aren't happy with your boss fight being so easy, is DM desire, not mechanics.
Coming up with an on the fly encounter is definitely not fudging. Like you say, reskinning, changing damage amounts and HP or AC is fine. Once the fight begins however, if you decide to change those stats mid-combat, that's definitely fudging.