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!
Nope, that won't work. Todd has updated to OS 10.10.3 and from the sounds of it, doesn't have iPhoto 9.6.1, which is the ONLY version of iPhoto that works with 10.10.3. Apple, in a ham-fisted effort to push people to use new app Photos, has PULLED THAT UPDATE FROM THE APPLE STORE. Yes, they've really done that.
So - the alternatives are to roll back the OS so that your old version of iPhoto will work (not what I'd recommend) or find an alternative. I'd suggest the alternative - get Lightroom. Great program - makes iPhoto and Photos look like amateur hour. (I never liked iPhoto to begin with - can you tell?)
Lightroom 6 has just come out. It's $149 USD and well worth it, IMHO. The subscription to CC for both Photoshop & Lightroom isn't too hard to swallow at $9.99 a month, though I loathe subscription model software and have continued using CS6 & Lightroom 5, and will likely update to 6 for the desktop, though am annoyed that there is neither an upgrade version OR an educational version. They've forced all educational institutions onto the cloud versions.
Another completely free alternative to iPhoto is Picasa.
And for photo editing, as opposed to cataloguing (which I think iPhoto did really badly), there are several alternatives much better than Photos. Pixelmator is a terrific program and a very reasonable $34.99 from the app store. GIMP is free - it just has a rather ugly user interface. Both of these are quite reasonable Photoshop substitutes in a lot of cases.
And here's another option:
http://www.lynapp.com - it has a 15 day free trial. It's $20.
So there are definitely superior alternatives out there and no reason to stick with something you're not happy with.