You'll want to send MIDI program change messages from PT, the same program changes that you send with for instance a midi pedal board. If you're already doing that in your guitar rig, well I guess it's more of a ProTools question, right?
If your question is about how the unit deals with MIDI program changes, it's explained in the manual, but there are several details to pay attention to.
For instance, to be able to do the program change at will on DSP A or B, you must set the MIDI option "Omni" to off. Then, sending a program change on the base channel will affect DSP A, sending it on base channel + 1 will affect DSP B.