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).

For the Audio-Unit plug-in, this seems to be impossible under MacOS 10.15.6 "Catalina", due to some unexplained persistence of the Audio Unit cache.

This repo contains the simplest code example I could prepare, to reproduce this issue.

tl;dr: The answer is "change the version number". See bottom of page.

JUCE provides straightforward mechanisms to define the number and configuration of input-output buses for a plug-in, which are illustrated by this simple example. The code is based on the JUCE Multi-Out Synth Plug-in tutorial, with three main changes:

1. I have replaced the SamplerVoice instances with a new SineWaveVoice class implementing sine-wave oscillators (based on the JUCE sine-wave synthesizer tutorial code).
2. In the MultiOutSynth constructor, I have replaced the static .withOutput() invocations with a new static function called buildBusesProperties() which adds one input bus, and one or more output buses, according to the value of a variable numOutputBuses, initialized from a #define set in each of the .jucer files as either 1 or 16.
3. I made appropriate changes to the isBusesLayoutSupported() implementation, so it accepts only bus layouts compatible with what was set up by buildBusesProperties().

The same source code is referenced by all three .jucer projects, which differ only in how they set the #define NUM_OUT_BUSES constant, the project name, and the Audio Unit subtype (Sout or Mout):

• Single-Sout.jucer sets NUM_OUT_BUSES=1 and AU subtype Sout.
• Multi-Mout.jucer sets NUM_OUT_BUSES=16 and AU subtype Mout.
• Multi-Sout.jucer sets NUM_OUT_BUSES=16 and AU subtype Sout.

The Binaries folder includes two compiled versions of the Single-Sout and Multi-Sout plug-ins.

• Debug-Version.zip contains Debug builds, compiled with Xcode set for Development Signing
• Release-Notarized.zip contains Release builds, fully signed and notarized.
• Release-Notarized-2in8out.zip contains a version of Multi-Sout built for 2 stereo input buses and 8 stereo output buses.
• SingleSout-VersionUpdate.zip contains three SingleSout builds with different version numbers.

I wanted to see if code-signing and/or notarization would make a difference; it did not.

What DID make a difference--all the difference in the world--was structuring the change as a standard version update. I compiled SingleSout with version "1.1" in 1-in, 1-out configuration, then again with version "2.0" in 1-in, 16-out configuration. I could then go back and forth between the two, and each time Logic Pro X (10.5.1, under Catalina) correctly detected the change. I then compiled the 1-in, 16-out configuration as version "1.2", just to check if there was anything special about the "major" version number; it appears there is not.

If you build the first two Audio Unit plug-ins (or use the prebuilt binaries), and install them on a "virgin" MacOS system, Logic Pro X will correctly identify that the Single-Sout plug-in supports only stereo output, and the Multi-Mout plug-in supports "stereo" and "16x stereo". (Logic does not present all possible options.)

However, if you then build and install the Multi-Sout plug-in, and remove Single-Sout (so there is still only one plug-in with AU subtype "Sout") Logic Pro remembers that the AU subtype code "Sout" is associated with one stereo output bus only, and I have not been able to find any way to reset whatever caching mechanism it uses, so it can recognize that the plug-in is now capable of supporting up to 16 stereo output buses. Things I have tried include:

• Log out / log in
• Restart MacOS
• sudo killall -9 AudioComponentRegistrar on the command line
• Delete ~/Library/Caches/AudioUnitCache
• Delete ~/Library/Caches/com.apple.logic10
• Delete ~/Library/Caches/com.apple.musicapps

The above measures appear to WORK with MacOS 10.13.6 and Logic Pro X 10.4.8, but FAIL under MacOS 10.15.6 and Logic Pro X 10.5.1.

Following a suggestion from Apple, I tested the same steps using Logic Pro 10.4.8 on MacOS 10.15.6 Catalina, and this works as expected. It is even possible to get Logic 10.4.8 to show the new I/O options by simply selecting the Single-Sout plug-in in the Plug-In Manager and choosing "Reset and Rescan Selection"; it's not even necessary to delete any caches. (If you do this, Logic will continue to use the old plug-in name "Single-Sout", but it will correctly load the the "Multi-Sout" plug-in.)

According to my Apple contact, the issue is not Logic Pro itself, but simply the fact that Logic 10.5.1 is the first to use "an OS service" (presumably AudioComponentRegistrar) to scan plug-ins. As my final set of experiments proved, AudioComponentRegistrar requires a version-number change to trigger correct re-scanning, and it works just as well whether the version number increases or decreases (so downgrades are perfectly possible).

In hindsight, this is hardly surprising, but I greatly regret that it took more than a week of extra work, and imposing on the time and goodwill of a friendly Apple insider, to learn this tiny but critical detail.