This mod rolls back Skyrim Anniversary Edition to Special Edition version 1.5.97
A big drawback to the Skyrim Anniversary Edition being an upgrade to Skyrim Special Edition, instead of a completely separate version of the game, is that the new edition can break your mods. Even if you didn't buy Skyrim AE, you may still experience mod incompatibility due to Bethesda's update to the Special Edition.
While the 'modpocalypse' might not be as severe as expected due to Bethesda giving the creator of Skyrim Script Extender early access to the changes made by the AE, there are still plenty of reports of broken mods circulating. As a result, some players with heavily modded games simply want to roll back to Skyrim Special Edition version 1.5.97.
Now there's a mod that can do that for you. The Unofficial Skyrim Special Edition Downgrade Patcher (should we shorten that to USSEDP?) will downgrade Skyrim from the AE version back to SE version 1.5.97. Then you can pretend the Anniversary Edition never happened, meaning your plugins, ENBs, and other mods should work just as they did last week.
It's a 3GB install, which sounds pretty big for a mod that doesn't include any actual game assets. "While we don't ship assets we do ship information on how to transform old assets into the new assets," reads the mod's FAQ. "Skyrim AE turned out to change a ton of stuff, so all the ESM files have data re-arranged, Update.ESM is patched into Skyrim.ESM etc. That means there's a lot of instructions. But 3GB is a lot smaller than the 16GB of game data that was modified."
It currently only works with the Steam version of Skyrim, and only with the English version. I would recommend proceeding with caution: I haven't tested this mod myself and can't confirm it works. There are also a number of posts on the mod's page at Nexus Mods from players who have had issues getting USSEDP to work properly, so if you're having trouble see if your issue is answered there. And to be safe, make sure you back up all your game files and saved games.
Meanwhile, there are others working on not just downgrading to version 1.5.97, but also keeping the new paid-for Creation Club features and quests intact at the same time. This Reddit post by DuxteR7 says by replacing executables and a dll from a backed-up version of 1.5.97 they were able to play the downgraded version of Skyrim with some of the new Creation Club features, like fishing, the horse armor, and one of the new quests added in Skyrim AE, right along side mods like SSE Engine Fixes (which hasn't yet been updated to work with the AE).
As reported in the thread, there are still mods that aren't compatible with some of the Creation Club content. "If you have a huge modlist with tons of graphics, new quests, gameplay mechanics mods it is best to not add AE content," says DuxteR7. "But if you use some minimal mods like SkyUI, Engine Fixes Unofficial Patch then it is probably going to be ok."
Again, if you're going to attempt a rollback, either using USSEDP or another method, make sure you back up all of your files in case things get even more broken.