Downgrading a Photos Library to a Previous Version of Photos with the Help of iCloud Photos
Photos 4.0 on macOS 10.14 Mojave is downward compatible and can open Photos Libraries created by earlier versions of Photos, iPhoto, and Aperture. But Photos is not upward compatible, for example, Photos 3.0 cannot open a Photos Library from Photos 4.0. Once you touch a Photos Library with a newer version of Photos, you can no longer access this library on a Mac with an older version of MacOS X and Photos.
It is essential to keep a backup copy of your Photos Library, before you try to upgrade to a newer version of MacOS X and Photos, just in case the upgrade is causing unexpected problems and you prefer to downgrade again to the previous versions.
If you want to share a Photos Library between Macs with different versions of MacOS X, iCloud Photos (Library) can help. You can sync your Photos Library with iCloud from devices with different system versions and keep the local copies on your devices updated from iCloud.
See this user tip: Use Photos and iCloud Photos on multiple Mac Computers
iCloud Photos can help you to create a downgraded version of your Photos Library on a computer with in older system and version of Photos. For example, if your Photos Library is stored on a Mac1 with Photos 4.0 and you want to access the library on Mac2 with Photos 3.0, do the following:
Upload the library on Mac1 with Photos 4.0 to iCloud Photos and download it on Mac2 with Photos 3.0 from iCloud to a new, empty Photos Library and you will have a downgraded library on your second Mac.
- If you are not yet using iCloud Photos (called iCloud Photos Library on older systems), sign up for enough cloud storage to hold your complete Photos library. (iCloud storage plans and pricing - Apple Support)
- Enable System Preferences > iCloud > Photos Options > iCloud Photos (Library) on both your Macs.
- On your Mac1 with Photos 4.0:
- Launch Photos, if it is not running on the Photos Library that you want in iCloud
- Make the library the System Photo Library, if it is not already enabled in Photos > Preferences > General "Use as System Photo Library". This needs to be grayed out: System Photo Library overview for Photos on Mac
- Enable iCloud Photos in Photos > Preferences > iCloud, and let the library upload to iCloud. Use iCloud Photos to store photos in iCloud - Apple Support
- On your Mac2 with Photos 3.0:
- Create a new, empty iPhotos Library. (Create additional photo libraries in Photos on Mac - Apple Support) The new library will be a Photos 3.0 library. Name it "Photos Library Mac2.photoslibrary". It needs a different name from the original Library, so you will not accidentally open it again with Photos 4.0, if you are keeping the library on an external drive.
- Then make this empty library your System Photos Library in the Photos > Preferences > General by clicking "Use as System Photo Library"
- Enable iCloud Photos Library in the Photos > Preferences iCloud.
- Wait for the Photos Library to download from iCloud to the empty library on the Mac2 .
- You will now have a Photos 3.0 photo Library on the Mac2 in sync with iCloud and the Photos 4.0 library used by your Mac1.
- The first syncing may take a very long time, depending on the size of your library. But the beauty of iCloud Photos is, that you can use the same Photos Library across devices with different system versions and keep them perfectly in sync.
If you do not want to pay for additional cloud storage, you can disable iCloud Photos once you have agan a Photos 3.0 library. Then cancel the iCloud storage subscription. It should suffice to pay for additional iCloud storage only for a month, if you just need it to downgrade the library.