Originally the 8 MIDI tracks used HSO, but I then changed them to use Vienna Symphonic Library.
This is when I realised I couldn’t just delete the track to remove the HSO association, so I created 8 new tracks and copied the MIDI data to them using Alt-drag (perhaps it copied more than the MIDI data?), and I was then able to delete the original tracks. However, the HSO VST track Instruments didn’t go away. I’ve a feeling I’ve seen something about copying MIDI data using different options but can’t find it in the docs.