I have a commercial plug-in product, for which I would like to offer a paid upgrade from the present stereo-only version (single stereo output bus) to a multi-output version, without changing the AU subtype code (so DAW projects saved with the earlier version will continue to load with the upgraded version).
This proves difficult under MacOS 10.13.6, but impossible under MacOS 10.15.6 “Catalina”. I have prepared the simplest possible illustrative code example and Projucer files in this new GitHub repo. The README file in the repo includes additional details.
I’m hesitant to take this up with Apple before seeking help from the JUCE team and community first, because it will be too easy for Apple to blame JUCE otherwise. Please note the code I’m presenting has been developed in consultation with Fabian Renn-Giles, who is aware of the AU subtype issue, but does not know of a solution. I am quite confident that my JUCE code is correct; this is strictly an AU/MacOS issue.