I tried the public beta program and while I liked it, in order to troubleshoot an endless wireless connectivity problem I've been experiencing, I've had to revert back to 10.10.3 just in case the beta was contributing. I have now reverted but receive the following error when loading the Photos app: I guess I either wait until 10.10.4 final is released or I find a way to hack open my library somehow. This means you are locked out of your library if you downgrade. There are surely times when a downgrade is 'legitimate' and 'should be supported', i.e. for troubleshooting if a problem is encountered with a specific version.