Converting older iPhoto to new OS

The free version of PowerPhotos allows for everything except the merge and duplicate finding.

Try the following;

1 - create a new, empty library with your Photos app.

2 - see if PowerPhotos can open and view the photos in the old iPhoto's library. If so continue on to 3.

3 - with the paid version of PowerPhotos merge the old iPhoto library into the new, empty Photos library.

4 - repeat Steps 2 and 3 for each of the other iPhoto libraries.

You can see what PowerPhotos can do in merging a library into another library by reading this User Tip: Comparison of PowerPhotos vs Photos in Merging One Photos library into Another

Follow léonie's suggestions.

Additionally, you may be able to use the free trial version of the trusted 3rd party app PowerPhotos ($40) to view the delinquent iPhoto Library--I'm not entirely sure what the free version allows.

Photos' Import method will not preserve the Events that you have in the iPhoto Library. The paid version can of PowerPhotos can merge the iPhoto Library to Photos keeping the Events as Albums-- if the Library is intact. It may be more successful than Photos. I'm sure that léonie knows more about this.

In the worst case, you should make a copy of the iPhoto Library and open it with the right-click menu. Then you can hunt around in there for pictures you can save. This may permanently corrupt the Library, which is why it doesn't open as easily as a normal folder, and it's why you only do this with a copy.

Which version of iPhoto has been used to create the libraries? The iPhoto Library upgrader is necessary for iPhoto Libraries creared by iPhoto 7 or older. But you cannot use the iPhoto Library on macOS 10.5 or newer.

And Photos for mac can no longer convert iPhoto Libraries to Photos Libraries at all. What you still should be able to do is to creste a new, empty Photos Library and use File > Import to import the iPhoto library into the new, empty Photos Library. Have you trief that?

Back in iPhoto days I had a hard drive failure that wiped out thousands of pictures after I had done lots of work editing them. For reasons I can't remember, what I had left were lower resolution edited copies --pretty good, but not original quality. The originals were in DVDs scattered all over the place, so my choice was high resolution originals or lower resolution versions that had all my edits--and I stuck with the edited copies. 15 years later I rounded up all the originals (a real mess with sometimes 12 or more versions of each picture,) and they're looking pretty good, now!

So--Good Luck!