Let's say you have a MIDI track.
Output is assigned to external rack synth.
 
Then you have a MIDI controller which does not not have program change +/- buttos.
 
Program can be selected in the MIDI track header pane from list but is it possible to change program number by keyboard shortcut (Previous / Next program)?
 
 

Your controller doesn't have buttons to do this because there's no standard MIDI message for increment/decrement program number. The synth might allow this via System Exclusive messages. If you have a very advanced controller, it might have the capability of storing and sending SysEx, but it would take some work to get it all set up. Probably easier to move the rack close enough to your keyboard to be able press the Inc/Dec buttons on the synth.  
 
 

 
Grin... I have to move it closer. Or use some kind of long stick to push the buttons. :D

There is a sendMIDI instruction in CAL which can send a patch/bank number or sysx. I have not used it though.

OK, so in theory I could create +/- buttons by using Duckbar if CAL can read the current patch number.

Another partial solution:
I added patch change command to the MIDI track's event list.
+ key increases patch number. However, it does not become effective until playback is started.
- key does not decrease but one direction is better than nothing...

Hopefully you can use sysx for +/- program. I do not believe CAL can know a patch number unless it is discovered from the existing MIDI data in the project.

I have not completely understood your target, but is case you want change the patch using MIDI keyboard/controller, I can try to make something using AZCtrl. For example:
* you just want send simple PC MIDI Message, 2 options:
  1) dedicate some finite knob, take whatever absolute value it sends and forward it to the synth as PC. Easy and possible now. Disadvantage: it does not save that information in the project
  2) use some buttons or encoder to inc/dec whatever is set in the Patch parameter for fixed track (routed to that synth) and in parallel send corresponding PC command directly to the synth. That way the change is "saved" into the project, so next time you reload and play from start, you get the same patch. That is not yet implemented in AZCtrl (in part of accessing patch parameter), but that can be done
* if the synth support Inc/Dec SysEx, some buttons or encoder can be configured to send that.

In the track Inspector click on the patch field and use Numpad +/-. Same for the Bank field. 
 
Or, click on the Patch field in the track header and use Shift+/- on the main number row.

dilletant
In the track Inspector click on the patch field and use Numpad +/-. Same for the Bank field. 
 
Or, click on the Patch field in the track header and use Shift+/- on the main number row.

... and then RTZ and start playing to apply the change... not extremely practical, is not it?