I am having the exact same problem although I was not upgrading phones or any other device. The photos just disappeared all of a sudden and I just don't understand what's going on. All photos except for the last 30 days or so were lost. They don't appear on my Mac, iPad or iPhone.
I hooked my iPhone up to the computer and synced them via iTunes. That way I recovered 1400 photos but I'm still missing the last 2,5 years worth of photos.
If your iPhone automatically uploads all photos to iCloud and other devices have been able to view them, one would assume that it should be stored in the cloud, right? The big question is, since this is obviously happening to more people, is this a possible server error or something of that nature?