How to insert or modify a stop change/combination stepper with a daw ?

I looked at your MIDI file and it is different from the ones I use. There are a lot of tracks with the same notes as if each track is to be mapped to a single stop and the resulting sound is to be the sum of the same notes with different single stops. Organteq works more like using one track (one MIDI channel) of many notes and pulling as many stops as necessary. In other word your MIDI files is more an orchestral rendering than an organ one, or maybe it is adapted for a different software organ. Usually what I use has three or four tracks (manuals and pedals) in the reverse order of the one Organteq uses so I just have to reverse the order of the MIDI channels and play it with Organteq.

I don't use Reaper, I use a version of Sonar (in a Windows VM on my Mac) to edit the MIDI file and insert Controller Codes using the MIDI event view at the appropriate place while scanning the file in score view. I suppose something like this can be done with Reaper.

The idea is to set up a MIDI mapping for these controllers, usually with saved Combinations. See the FXP for BWV 768 I had posted:
https://forum.modartt.com/file/a3rr6sxz

I always use the four keyboards mapping.

Don't know if this helps...

Thank you both for the advice you have offered.

After testing the ease of use of Organteq 1.04 on the following 3 daws: Cubase 10.5 Element, Bandlab Cakewalk (ex Sonar) and Reaper, I finally stayed on Reaper.

Insofar as I have neither an organ console, nor a sufficient command of Organteq to change the stops during the game and where I would first like to understand the possibilities (and acoustic limits of the stops of the version current from Organteq), this weekend I discovered on Youtube an extract from the festive "Toulouse les Orgues" 2007 with a transposition of part of the 1973 version of Tubular Bell (Mike Oldfield), played by Virgile Monin ( author of part of the Organteq demonstrations on the Modartt site), on the magnificent Cavaillé-coll organ in the Basilica of Saint Sernin.
I couldn't resist the temptation (despite my inexperience with the organ) to try a quick comparison (1 day only) by trying to map the 41 tracks of an existing midi file (JD Mack / link: https://tubular.net/midi/TubularBells1.mid ) with the current version of Organteq + pianoteq for the 1st piano track and the tubular bells track. For the occasion, I abandoned my usual audio format of 96k / 24 bits to fall back on 48k / 24 bits so as not to make my laptop too noisy in core i9 9900k.
(here without limitation of polyphony always at the maximum of 512)
The good surprise is that it has not exceeded a consumption of a few peaks at 45% of cpu (well distributed over the 16 threads), and less than 30% most often. Reaper loaded simultaneously the engines of Pianoteq and Organteq (+ the engine of Halion6 & Tokyo Down Lab - Nova Ge Dynamic parallel equalizer) only for 2 tracks that I added in dubbing, reserved to "simulate" (very roughly) the stops of 32 'unfortunately missing in Organteq (main flute bass 32' and bombarde 32 'of the organ of Saint Sernin, actually used in the video of the October 2017 festival in Toulouse) - only for notes below C 32hz -, the other notes and on all the registers being much better with Organteq.
The result of this "beginner" draft "(lack of time - very much to rework) is as follows:
Link: https://forum.modartt.com/uploads.php?f...xtract.mp3
I am amazed at the good integration of Organiteq and Pianoteq with Reaper, even with a significant number of tracks, even if I was intrigued by the presence of abnormal distortion on the piano track of Pianoteq (Bluthner with my Bosendorfer 200 fxp in standalone is without significant distortion), distortion perhaps due to the presence of a midi track with constant velocity, set at relatively high velocity to bring out the instrument otherwise drowned by the organ (unlike tubular bells pianoteq which do not suffer distortion in 1st approach)
As was the case in my previous test, I note, in the presence of a number of simultaneously played channels: total of: 285 stops organteq (distributed over 34 instances, if not shared? Including 8 types of instances with a config of different stops)
+ 2 Halion6 organ tracks and 2 pianoteq tracks: (in polyphony 256 for pianoteq)

Of course, building this quick example with this DAW, as Gilles would point out, is far from the most effective approach for managing instances of Organteq ...

The result of the transposition of Tubular Bell in 2017 on the real organ of Saint Sernin (slightly raised in bass below 32hz to "try" to compensate for the probable weakening of the microphones, and a little that used for the restitution at l 'listen) is as follows:
Link: https://forum.modartt.com/uploads.php?f...uation.mp3

With the corresponding video on the organ of Saint Sernin on this link:
https://youtu.be/22H7M3-R7DQ

The first (partial) feedback I get from this essay on the pianoteq 1.0 timbre
is that bass management can still be improved in Organteq (Example: video between 7 ': 21 and 7': 50), especially on the bombarde, which I (personally) find more "fleshy" in reality, can be also the "bourdon" which could have a little more breadth,
and finally, the already reported lack of 2 stops of 32 '.

There are of course other differences, but to clearly highlight them, it would have taken time for the amateur that I am, for:
- try to identify the combinations of games used on the organ of Saint Sernin in the 2017 video
- try to find them (or approach them in part with several instances of organteq and some filters)
Correctly balance Tubular Bell's tracks in Reaper .. (some of which can be added - piano for example)

The authors of the 2017 video would have been the ideal people to build such a comparison with Organteq ...

Bruno