After you open the midifile, press F1 (GM PART), you could use the page #s to scroll each channel, or F10 (16 PARTS). You can chose any onboard sounds except programs. You must do this after the midifile starts playing, because that's after the initial sysex kicks in.I usually start the file, hit pause, make the changes, unpause, and stop; but make sure before you stop the file, that you are still on the General Midi page. Hit "save/enter" and save the file. I have done many songs this way.
Hope this helps.