I recently changed from Windows to Mac.
When opening files made with the Win version of Dorico, the following happens:
Paper format in the print dialog changes to “215x345 mm”, although the files were all saved in A4 format. In my prefs, I have set the default paper size to “International”.
Chord diagrams have changed. What is the official way to set the defaults here? I tried marking the ones I want with a star in the “Edit chord diagrams” window, with no success.
You shouldn’t find that chord diagrams in existing projects have changed, but perhaps what you mean is that when you add new chord diagrams in new projects on your new computer, you are seeing different choices because your saved defaults are not present on the new machine?
You should copy the “Dorico 4” folder from %APPDATA%\Steinberg\ on your Windows computer to /Users/your-username/Library/Application Support/Steinberg on your Mac to bring all of your user-specific configuration across.
That folder did not survive the Linux installation on my old machine 
I am not talking about new diagrams, but just opening existing files, some diagrams definitely have changed. Flagging the ones I want as favourites does not work.
I’m surprised to hear that, but I guess it’s going to be difficult to prove since you presumably no longer have access either to the original computer or to the user data folder from it.
Nope, that folder is gone forever.
Is there a way to set a default diagram for a certain chord symbol to always appear the same?
Not as such, no, but certainly those chord shapes that you have edited yourself and then saved as default will be the ones that are offered to you ahead of the factory-supplied ones.