Photos App in Catalina renamed ALL my original photos in the library.

The structure of the Photos Library has changed considerably. It is now more difficult than before to extract the original image files directly from the library package. To save your originals with the original filenames use the command "File > Export > Export unmodified original". as long as Photos is working well and you can launch it, you can always get your originals back by exporting the unmodified original. This is usually very quick, even for large image files. we are not supposed to access the originals directly inside the library package, so it should not matter.

Only, you will have. hard time to access the originals, if your Photos library should get corrupted and Photos cannot open it. If Photos should not be able to repair your library, there will be no easy way to recreate the library by extracting the originals from the package contents. I strongly recommend to take precautions for the rare event, that Photos may be unable to repair the library and you should lose the access to your photos library. Save your originals to an external volume before you import photos to Photos. Save the originals of already imported photos by exporting them with "File > Export > Unmodified originals". It has always been a good practice to make backup copies of the originals before importing them to any DAM. Aperture had a built-in feature to make backup copies automatically before importing them.