GMforPowergamers
Legend
depends on what your goal is (and by your I mean the general player and DM) if your goal is a fun climatic encounter, and the PCs would have a board encounter that lasts less then a minute out of game and be a downer... then it isn't cheating it's making the game fun.The DM swapping out spells mid combat: this definitely feels like cheating to me. The cunning PC traps the enemy spellcaster behind a wall of force. The DM, annoyed at the player's cleverness, swaps out one of the enemy's spells for Disintegrate.
It doesn't matter that the players would never know, it goes against the unwritten rule to give the players a fair fight.
Swapping spells and abilities before the encounter: absolutely fine, I do it all the time.
I can have it happen twice in a campaign and think it cheating once and not another...
we have a wight warlock in a dungeon and the PCs shut it down in 1 action and everyone cheers it was a great quick thinking moment... if I do it there I would consider it cheating... but the opposite, if I DID have the counter (disintegrate in your example) and at that moment mid play I decide "nope he has no way out, here is your easy XP and way to go" isn't cheating. Later in the same dungeon we get to the ELdritch Knight lich/Deathknight hybrid that has been the quest for 4 months out of game and a year and a half in game... it is the climax of the game and someone pulls a 1 round end... and everyone just pauses like 'wait, is that it' and I then give him the counter... no that isn't cheating.
As teh DM it is (IMO) my job to make the game fun. that wight warlock was fun to one shot... the quest ender not so much.