A problem with using the iphoto library upgrader tool
After a fair number of tears thinking apple had stolen a decade of my photos, I found a work around too which might be the exact same thing jvans said, but brought down to my novice level.

Right click on the iPhoto Library that won't open, and go to "show package contents"---
inside there is a folder called "Originals" that holds the actual .jpgs of the images
Copy this entire folder out onto the hard drive. Then, you can import the whole folder into your new "iphoto" or "Photos" library, or just have them for future use on your hard drive. Not ideal, but much better than losing your entire library.