And when you fooled ALL of the directions?
First, go to the App Store, Purchases tab, and make sure iPhoto is in the list of purchased software. If it is not, this will not work, so don't bother continuing.
Just as a precaution, copy iPhoto from your Applications folder onto an external disk or USB stick and eject that drive.
Delete iPhoto from your Applications folder. Make sure there are no disks mounted that have a copy of iPhoto on them. Empty the Trash.
Make sure no other users have iPhoto open in the background. It is best to have all other users logged out.
I ended up having to Restart before I could clear iPhoto from the App Store memory (it was still showing Update on the button in Purchases).
If it is still showing as Update, try searching Spotlight for iPhoto. There may be a copy on a mounted external disk that it is identifying.
You should now be able to go to the Purchases tab and find iPhoto with the Update button labeled "Install"Click "Install" and it should download the current version which works with Yosemite.
When I first tried, after authenticating, it posted a 503 error and asked if I wanted to Retry. When I clicked "Retry," it started downloading.
It's about 1.2 GB, so it takes a little while to fully download.
LN