It changes prime material people from being correct to being wrong. That's a huge change.As I believe I noted above, your average "Prime Material" character (in a Planescape game or otherwise) is COMPLETLY UNAWARE that Demons and Devils have any other names than "Demons" and "Devils"! The fact that the Demons and Devils (and other characters "in the know") know better in NO WAY changes this!
Qv: in Marvel Comics the vikings are wrong about who Thor and Odin are - they thought they were gods, but really they are something like aliens from another dimension.
If I write a RPG in which vikings are correct in their religous beliefs, and then you rewrite that along the lines of Marvel Comics, you've dramatically changed the game.