It may help to migrate through the various iPhoto versions that are between your copy & the last one in the App store. The trouble is that you cannot purchase older versions from Apple, so you are hoping that Apple will upgrade from a very old library format. The longer you leave it the less likely it is for Apple to test that migration path. I noticed 10.6 started to cause issues with migrations during 10.9/ 10.10.
Personally I'd create a test installation on an external disk. A fast Thunderbolt, Firewire or USB 3 external disk can be enough to see how it performs. USB 2 is a bottleneck compared to an internal disk so don't expect that to work as well, but it can be sufficient to test with.
A test install isn't too useful if you can't migrate all your data onto it & perform the upgrades. Using the original partition with an updated OS will irreparably alter the iPhoto library. It's a good idea to eject the internal disk too when testing, to avoid writing to it or accidentally opening data from the wrong disk.
My upgrade preference is to avoid migrating Applications (only choose User data), especially when jumping over multiple OS releases, the apps can be installed & set up according to the new OS features.