Changing VSL's in an existing file

Whenever I change the VSL in an existing file, I get no sound in playback until I reassign all the instruments one by one in Setup mode. Strangely, all the instruments are already listed correctly in both Setup and Play mode but I still have to go through this time-consuming task which shouldn’t be necessary. What am I missing?

When you say “VSL” I guess you mean something generic along the lines of “virtual sound library” rather than specifically “Vienna Symphonic Library”? You should find that the previous playback template is automatically restored when you open an existing project. But if not, you should be able to reset it by going to Play > Playback Template and choosing the default playback template.

You’re right about my use of VSL. Sorry for the ambiguity. And I had indeed overlooked the Playback Template menu item. When I select Playback Template, the default is already loaded and I just need to select Apply and close, so I’m still at a loss as to why this extra step is necessary.

If you find that sounds are not loading automatically, then that is because you have made manual changes. Dorico will load sounds automatically when you create a new score or apply a playback template. If you subsequently make manual modifications by loading new plugins or changing the routing of tracks in Play Mode then Dorico will stop loading sounds automatically. The reason for this is so that it doesn’t accidentally overwrite any changes that you have made yourself. As a general rule, applying a Playback Template should usually get you back to a known good state.
If you find yourself wanting to load your own plugins a lot then I would suggest looking into Custom Playback Templates: https://www.youtube.com/watch?v=PLCOhCXRjEg