Welcome to the growing club of 'Change for what?!' club.

While I never used iPhoto, I've heard lots of folks upset about the change to Photo.
You just might be able to force the OS to do your bidding as to what app opens what file, however.
1. Find any image file you want to have opened in iPhoto by selecting it in Finder.
2. Use the "Get info" menu item (command & i).
3. Near the bottom of the Get Info window is a section called "Open with:".
4. If that is all you see, click the disclosure triangle.
5. You should now see the name of the app that the System will use to open that type file.
6. Click that label/button/pop-up list and see if "iPhoto" is in the list, if not, at the very bottom is a "Choose..." item that will let you search for the iPhoto app.
Hope that works for you!