So from my testing with the beta, then having to roll back and losing 100GB of music, be aware that if you have an original Apple Watch, it’s not compatible with iOS12.
The just-release Watch OS 5 is compatible with Watch 1: https://www.apple.com/watchos/watchos-5/ so w 1 should be compatible with iOS 12
When they released the Series 2 with better processor, GPS, brighter screen and better waterproofing, they simultaneously killed the "Series 0" and launched the Series 1 as the low price model. It got the new processor but not the other improvements.
That leaves the Series 0 as the only one with the original crappy processor, and support for that has been dropped in watchOS 5.