I do not like the new layout for iPhoto in Yosemite upgrade - can I go back?

I dont think iPhoto changed from Mavericks to Yosemite, although it does require an update.

Also so note that Photos (new in 10.10.3) is not iPhoto. It is a replacement for iPhoto and Aperture, but iPhoto still shoul be in your Applications folder.

If you didn't update it to a version compatible with Yosemite, see here: Can't Update iPhoto because it is not available in the App Store