Yes, I make changes to encounters while they're in progress. The main reason for that is that I don't assiduously prep everything, and that requires improvisation during the game. I have a monster stat block in front of me, but I don't list every piece of equipment that monster has on hand. If they get disarmed and it makes sense for the monster to have a dagger, there will be a dagger. I have barebones descriptions of areas in dungeons, without a full accounting for all details and dungeon dressing, so I often make it up as I go, and that can affect how an encounter unfolds. If there are reinforcments arriving on the scene, I might time those randomly or might chose a dramatic moment.
I don't outline monster tactics in detail in advance, since those usually get rendered moot after a few PCs have their turns. So I change up tactics on the fly, which has a much greater effect on the outcome of an encounter than lowering a monster's hit points or changing a crit into a regular damaage roll would ever have.
Sometimes during my prep time I think of cool and awesome stuff. Sometimes I don't think of the cool and awesome until I'm in the heat of battle, so I use it when I think of it.