Fifth Element
Legend
I don't see this as a meaningful distinction. If as DM I am in charge of deciding what monsters are there, I don't see a meaningful distinction between deciding it last week and deciding it just now.1) You are playing the game when a player says, "I bet its mind flayers." You inwardly think, "Yeah, mind flayers would be really cool." and change the trolls to mind flayers. This is illusionism.
2) You prepared two weeks of material. In between sessions, you see a billboard and you think, "Mind flayers would be cooler than trolls. I should do mind flayers instead. This is not illusionism.
If I make a last-second change to mind flayers based on my own whims, not on something a player has said, is that illusionism?
You've said that the source of the inspiration to change doesn't matter. So is it entirely the timing?