The issue might be that autosave might’ve been triggered, which saves your assets into the new unreal version. Once that is done, those assets can only be opened with that version or unreal or even newer. You can use my plugin (https://www.unrealengine.com/marketplace/en-US/product/asset-downgrader) to go back a few unreal versions, all the way back to 4.27 with certain limitations (for now). Let me know if it works for you.