You cannot downgrade a photo library, since downgrading overall is unsupported. Your files are not lost, however, since the Photos library is a bundle that still contains your originals. You have the following options:
If you use iCloud Photo Library, create a new Library and configure it with iCloud. It will pull down your photos.
If you have access to a machine with Big Sur you have two routes:
- Configure that machine with your iCloud and set up iCloud Photo Library. The photos can be uploaded, then your Catalina machine can download them.
- If you don't want to set up iCloud Photo Library, use that machine instead to export the files. Reimport them to a new library on this machine. Verify the process with a small batch to ensure quality.
If you cannot do either of these and must be on Catalina, you must open up the photo library and extract the originals manually. If you locate the library in Finder (Usually located in the Pictures folder) and right click it, click Show Package Contents. There is an originals folder that contains all your unmodified images. You should be able to import this folder.
These options are listed in order of best to worst. Only use approach 3 if none of the other approaches are viable. Even if you don't have a second laptop, considering installing Big Sur on a second volume (here are instructions for the beta, but they apply for the regular release as well) and using that to upload to iCloud Photo Library or Export.