Sorry a regression path is a process to move back to the previous state. When my iMac upgraded to Photos it migrated the library to Photos. I was wondering if there was a process to migrate back until the Photos application is a bit more useful or I find another application?
The update to MacOS X 10.10.3 Yosemite created a second photo library for Photos, but your original library will still work with iPhoto. Only, you need have iPhoto 9.6.1 installed. If you have the correct version, simply double click the iPhoto icon in the Applications folder and confirm that you want to open iPhoto and not Photos.
If your iPhoto version is an older version and the icon crossed out as incompatible, reinstall from the AppStore, if you can.
Apple stopped selling iPhoto, but anyone who bought iPhoto from the AppStore or updated it to a version sold at the App Store (iPhoto 9.4.5 or later) will be able to reinstall from the App Store by downloading from the Purchases tab.
If you have any version of iPhoto, that has been updated from the AppStore and is associated with your Apple ID, try to reinstall from the "Purchases" page.
- Unhide iPhoto, if it is hidden on your Purchases page (See this link: Mac App Store: Hiding and unhiding purchases)
- Trash the incompatible iPhoto version, but don't empty the Trash.
- Reload your Purchases page at the App Store with ⌘R
- Then download from that page.
The step "Trash the incompatible iPhoto version, but don't empty the Trash." must not be skipped. If Spotlight sees a version of iPhoto installed, the AppStore will try to update instead of reinstall, which does not work.