Help with Version

I have a problem. i have two accounts for two servers, one vanilla and one modded. So on my vanilla account server, i decided to experiment and tried to change the version there and back. It worked on switching to Snapshot and back, but I went and made the version 1.15.2 Forge, it saved the world but if I change back to vanilla it says the world would be deleted. Can you please help me? I don't want to delete the world but I have a freind who is only learning to play Minecraft and he's a noob. he says he's not ready for mods, so can you please change it back to vanilla please? Please

Thanks!