Insert patch change commands in the relevant MIDI track at the point on the time line where you need the change to occur.
If you need help learning how to do this, two sources of good information.
The Roland Book on MIDI and of course the Reaper user guide. General principles as per Reaper from the user guide, Nut and bolts of why and how from the Roland booklet.
Oh and you will of course need the MIDI implementation chart for the VSTi you are using to know what code to send.
Have fun!
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