There are several ways to accomplish this (and also alternatives)
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As documented in the user manual, you can enable the “Pass unused PC messages” option which means that any incoming PC messages that are not associated with a rackspace (actually variation in a rackspace) will be passed through into MIDI In blocks and so to any plugins to which they’re connected.
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You can map a widget to the PC parameter in a MIDI In block and send a program change to a connected plugin that way
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You can create a scriptlet plugin and connect that to a synth plugin and send PC messages to it
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You can create a GPScript and use the LoadGPPreset functions to load new presets into a synth plugin
Note however that
- many plugins don’t support the program change message at all and
- changing presets isn’t going to be instantaneous