I don't think the update will work. I've seen an update for iPhoto in the App store before and it usually gives you an error message that the item is no longer available (tho it has been a long time since I last saw that update). If you ever delete iPhoto, the update will disappear from the App Store.
I didn't think there was a way to continue to use iPhoto, but there appears to be a work around I found in Apple's Discussions forum. The basic idea is to delete then replace the iPhoto app by using the purchased list in the App Store. Theoretically, that version will be an updated version that will work. If it doesn't, work: No harm cuz it hasn't worked anyway. Check if your purchased list includes iPhoto and which version it downloads before attempting to remove the iPhoto app. Once iPhoto is the Trash, don't empty the Trash. You can put iPhoto back if this plan doesn't work and maybe someone will come up with a different plan later. And don't worry about losing the old library. Deleting the older version of iPhoto, or just moving it to Trash, will not remove the iPhoto library which usually resides in the Pictures folder of the home folder.
If that plan does work, the question then becomes how do you get pics from the Photos app to iPhoto that you added since you switched to Photos? Drag and drop will likely work even if you have to drag to desktop then drag to iPhoto. Try dragging pics directly from Photos to iPhoto first.
