I am looking for software that can convert a MIDI file in organ to a string or trumpet etc. automatically. Is there such a program?
Any help is appreciated.
Most MIDI sequencers will be able to do this. Generally, I would expect that you will need to do something like this:
load or import the MIDI file into the MIDI sequencer
play the MIDI file and see if it plays as you would expect in its original state. You might have to alter the sequencer's output or MIDI mapping settings to make sure it is playing through an appropriate MIDI synthesizer, such as (on Windows) "Microsoft GS wavetable synth" - or, of course, something better!
Change the voice settings of the tracks for which you want to change the sound. Your sequencer may help you by telling you which numbers correspond to which voices, or this list may help: https://www.midi.org/specifications-old/item/gm-level-1-sound-set
Save or export the file as a General MIDI file
The file should now have different "program change" data at the start, which will cause the notes to play as a different sound.