Question: farewell to OS X, welcome to macOS
Finally installed High Sierra, skipped Sierra, from El Capitan, the last of the OS X.
First I made a Carbon-Copy-Clone of my internal hard drive to an external USB. Then downloaded the High Sierra installer from App Store.app After downloading the ~5GB installer, the process went fairly smoothly, took about 30 minutes or so.
Up and running macOS High Sierra now, but waxing nostalgic for the original "Mac OS" <9/Classic & Mac OS X but now it's Welcome to macOS :-) Still have El Capitan on my external USB drive if I need to boot from it for any reason.
Farewell Mac OS X!