Expect Many Devices to Move from 8.0 to 9.0
Samsung's finally deployed Android 8.1 Oreo and we've been getting questions about its release from those still running Android 8.0 Oreo.
At this point, if your device is already running Android 8.0 Oreo, expect your device to remain on Android 8.0 Oreo until you get upgraded to Android Pie.
The Galaxy devices currently running Android 8.1 are new devices. The devices that will likely get upgraded to Android 8.1 Oreo in the future will be coming from Nougat, not Oreo.
Samsung did something similar last year with Android Nougat where the only devices that received a version change to Android 7.1.1 were 1) new 2) previously running Android Marshmallow.
So if you're currently using an older device that's eligible to receive an upgrade to Android Oreo, there's a good chance you'll get Android 8.1 down the road.
If you're currently using a device like the Galaxy S8, your next major version change will probably be to Android Pie.