I have several old, old iPhoto libraries archived on CD. I am doing a project that requires some pictures from the early '90s. "No problem" I say. I pop one of those archive disks into the drive, iPhoto opens and…
Oh Crap! But no worries. Clicking on Learn More loads this page which contains a link to download iPhoto Library Upgrader 1.1. This app, which runs on Snow Leopard and later versions of OS X, converts an old iPhoto Library, copied to your Photos folder, into a format that later versions of iPhoto can recognize. I'm currently running iPhoto 9.6 under Yosemite, and it turns out that more upgrading is necessary. Fortunately iPhoto handles that part.
Voilá! Digital Dark Age averted.