Hoo boy…we havin' some fun now, right?
This is the exact reason they came up with GM to begin with – to try to provide at least a basic standard to resort to so you could get usable output from a different MIDI device without having to reprogram which drums / patches were in which places.
just stating the obvious: you need a note mapping (midi module) that replaces an incoming GM note with the one that fits the target kit.
That's absolutely correct. The trouble is… you said;
The midi files are programmed in the GM note format (they were all made with the default Quicktime OMS thing) but now I want to send all of these to different modules.
If you want to resurrect 20-30 old files to use with multiple new samplers / modules, you're just going to have to do the work.
If all of the files are GM, you can just use one "new" module / device / sampler mapped to respond to GM placements. Drummers pretty much use one drumkit, right? If you have / need very different kits for very different genres, you'll just have to set them up.
This ain't nothing new. You're only asking because you have a "big batch" problem. Lots of songs / files and you're looking for a shortcut. You
know that generally, the more shortcuts you take, the more mediocre the results you get.
Just dig in. It will take less time than you think once you get a flow going. Since as you say, the files notes already conform to GM, don't move them. They're all now in similar locations - the last thing you want to do is mix up
more stuff. Create maps for the devices you want to use now and make them respond properly to what's there.