That sounds like a solution that changes the original file, Bruno, unless there's a way to do this in one step (which I think Claude is asking for).
Extreme transposition (5th up, 4th down etc.) has always been an issue with MIDI (and audio!) and short of a flag to tell the transposition to go 4 down on some tracks and 5 up on others (can't remember seeing anything capable of doing this as a one step process) it's pretty much a case of taking the songs that tend to need this done to them and permanently transposing the parts down an octave that chipmunk up when set up to a 5th or so. But you'd have to give it a name so you don't pick that file for small transpositions!
And yes, should you choose that path, MIDI KIT would do the job well.
Alternatively, you could use a DAW/Sequencer to edit the chipmunked Parts, and do the transpose there, which would allow you to take however many notes that are t'posed down too much and pop them back to original pitch.
I have a fairly deep baritone, and seldom find anything in pop music that doesn't need transposing down for my voice (and the older I get, the further it has to go!) so I've come across this issue a lot. To a certain degree, transposing and entire track up or down to the extremes (5th, 4th) seldom is satisfactory, as there will always be some notes that now end up too low (or two high).
What I tend to do is transpose a track BOTH down the 4th and up the 5thm then cut and paste between the two Parts to keep the instrument within the normal range of the real thing. And that's something that is basically just a bit too advanced for most software (particularly if you want to try and make the line not sound too jumpy).
TBH, unless your MIDI files have a VERY consistent channel assignment (guitars always on the same track#'s, strings, horns etc.) it's hard to come up with a macro to do the job, and if you are going in to the file to find sounds in the first place, might as well do it in a DAW and create that 'Extreme Tpose' version while in there! Probabnly five minutes editing per song once you get a flow going...
The problem with extreme pitch shifts is that many sounds certainly chipmunk if sent up the octave, but you often get them turning to mud if sent down the octave. It depends on the range of the part in question, but if a part is played within the natural range of an instrument, from standard low to standard high, dropping an octave can push guitar parts down into bass parts, or strings and horns down into very unnatural areas etc., and you end up with a part that's just as bad (if not worse) as transposing up too much!
I know it's a far less simple solution, but my solution of editing the transposed up AND down parts into one single part with just the too high stuff taken down, or the too low stuff taken up is by far the more natural solution.
If you asked a real acoustic musician to play a part in a key as far away as a 5th or 4th, he would not transpose the entire part note for note in the new key, but create a new part that smoothly moved within the note limitations of the instrument.
A quick individual track transpose might be a handy idea if you have no time to edit a track (and I certainly wouldn't want to dissuade Bruno from adding it if it's an easy thing to do!) but it isn't quite the panacea you might think it will be. Taking things down a 4th or 5th can cause almost as many problems as raising it a 5th or 4th...
I get exactly what you're saying but I have done this in previous programs and it worked fine. Its not used all the time, and I agree its not the best way of doing it, but sometimes we dont have time to re do a whole track, or sometimes you get an person who wants to sing on stage but wants it in a different key, It would be a handy feature to have to transpose each channel up or down individually.
It it can be incorporated in the program, it would be great. :)