Well you can't change the file association for a file type/extension if the file type/extension does not show up in the app settings in the first place! You can't change a setting that isn't there.
Eventually had to install Photos from the MS Store, and then force the Photos app into a repair...
There **IS NO** "Change" button for "Open With" in the Properties dialog. Microsoft in their infinite ARROGANCE has decided you are not allowed to use anything that Central CCP Command has dictated you should use.