I did a clean install of the GM off a USB stick. I had iCloud Photo Library enabled, with my entire library uploaded in Yosemite. My Photo Library was on an external drive, so absolutely nothing about the library changed during the upgrade. Nevertheless, when I opened Photos in El Capitan, it detected my library with no problem, then proceeded to upload all 50 GB of photo library to iCloud. It doesn't appear to be duplicating anything - it's just replacing every photo in the cloud with a new, identical copy. I'm fortunate that I have a fairly fast connection and no bandwidth caps, but if it was say, my parents, they'd have to split the upload over several months to avoid getting charged huge overages. Has anyone else seen behavior like this?