Sometimes you have story-related fixed encounters with some creatures where you cannot change the creature type. If needed because the encounter would be too low/high for your party you buff the challenge rating of the encounter, not the creature type. Pretty sure you already found that in some PNP campaign you already played, you just don´t know it because it´s something that only the DM knows.
I mean, if you are supposed to raid a greenskin fortress to kill the goblin king you cannot change it and create a fortress full of lizardfolk and make the goblin king a beholder just because...
Last edited by _Vic_; 09/05/20 09:46 PM.