They are available in the face gallery in the Watch app on the iPhone and can be added from there. I don't think it adds the new faces to the running set on the watch automatically.
Yeah many have to manually add them, a few had them pop up automatically I think.
You can open APPLE WATCH app on the phone and choose the new faces.
Or Press the watch face down to get other faces..move over to "NEW" (+).
Press that and scroll - or use the crown - to get the new faces.
If you don't like the default Kaleidoscope faces, I do like the new option in iOS11 of taking any photo and immediately have the option to add it to the watch. I took one that simply shows a thin line against a black background, saved it.
Added it to watch face using the "Radial" animation. It gives a cool animation of a thin circle "pulsating" Little different, gave the face a cool effect.