

Knowing that, I should be able to see the previous version in the App Store as outlined in this answer. I have previously upgraded my personal computer as follows: El Capitan - Sierra - High Sierra. I have a MacBook Pro running El Capitan that I need to upgrade to Sierra 10.12 (not High Sierra) for development reasons.It's not a major update, so the developers don't fully disclose what exactly has changed in the operating system.

In March 2017, Apple released an update to the latest operating system for Macs - macOS Sierra 10.12.3, which full version can be downloaded here. MacOS Sierra Full version Free Download.When you first open Photos on your Mac, your photo library is upgraded. If you didn’t update to OS X 10.10.3 or later, the Photos app is installed automatically when you update your version of macOS. I wonder if it might have something to do with other software I have installed on Sierra, my High Sierra installation is 'clean' still, I might have other versions of Quicktime, ffmpeg, and whatever, on my Sierra installation. The videos still play in iPhoto if I boot Sierra. However, in order to obtain that version you must have downloaded some version of iPhoto 9 from the App Store prior to the system upgrade. When you upgraded the system to Sierra the only version of iPhoto that will work is iPhoto 9.6.1. As beneficial as High Sierra was, the main issue that Mac users had was the fact that, after upgrading from macOS Sierra, many users found that their iPhotos were randomly going missing from their iPhoto photo libraries for no obvious reasons. It was released back in late 2017 and was named after the High Sierra region of California.
