I know this is tagged as solved but just in case my experience helps others...
I am in the UK so this all relates to UK apple store. I upgraded my mac two days ago to Yosemite 10.10.3, Apr 15 2015. Beforehand, vaguely concerned about iPhoto, I checked that you could still use it in Yosemite by looking on my husband's newish mac book air that still showed it working - with hindsight, obviously should have looked into it in more detail, but thought it would go as smoothly as previous OS upgrades. Didn't spot that he had the latest iPhoto whereas I had 9.5.1.
So, then when I could no longer use the old version in Yosemite but could also not upgrade it through the App Store, I found this thread. Like tedster998 step 3, I went to the App Store "Purchased" tab and found the "Update" button, clicked it, it whirred for a second or two but seemed to do nothing. I then went to the App Store's "Store" tab and spotted the "Check for unfinished downloads..." option. I clicked it thinking maybe this would finish the update process - iPhoto starts downloading for 20 minutes. The button switches to saying "Installed". (I think I have done it!)
Now, here I am not sure what had actually happened - I think checking for unfinished downloads must have picked up an unfinished download of an old version because, after my Restart and search for iPhoto using Finder, it says it was modified today but still appears to be version 9.5.1 (as it had been throughout).
However, I then try the steps all over again. I move that version to Trash again, go back into Purchases, and now it presents iPhoto with a new "Install" button (which I hadn't got before even though I'm pretty certain I'd moved the old app to Trash that time too - curious). Click that again for another 20 min install, Restart mac and, hey presto, iPhoto.app is installed and shows 9.6.1. And it is back in my toolbar with its old tropical beach/camera icon.
Basically, my lesson from this is 'if at first you don't succeed, try again'!
Only a few hours work searching the internet and all these forums - now, wouldn't it have been easier, Apple, simply to have kept it available to upgrade in the store? Thanks everyone in the thread - would never have got there without you! 🙂