Halion has started opening exclusively in the Generic Editor with no option to switch to the Plug-in Editor. You can see in the pics below that Retrologue has the expected menu option while Halion doesn’t.
How This Occurred
Apparently I had some older libraries that needed to be updated (and hadn’t) to work with the new licensing system. When I tried loading any of those I got an error saying the library wasn’t licensed and the GUI switched to the Generic Editor. I’ve since updated the libraries which now load fine and also reinstalled Halion. But I’m still stuck with the Generic Editor.