why did my calamity world suddenly convert to master mode and say its a large world???

[professional_idiot]

Was playing Calamity on a classic mode world until when I save and exit, when I returned I saw it said "I am Cthulhu incarnate" how did my world suddenly convert to master mode??? I didn't even beat expert mode on vanilla (yet) so there is no way I can do this. Is there any way to revert it back to classic? and if anyone knows, what the hell caused this to happen?

[Geekbar Enthusiast]

from what ive seen this is caused by the overhaul mod ive never played with it so idk how you would change it back but the mod page might tell you