Disklavier

Hi.
I got a little C110 disklavier Mark III, and a Mac G4 with Mac os X 
10.3.9
I want to modify midi files from the web which doesn't make play the 
piano (by moving on the first of the 16 pists), I don't find what soft 
can do that (and also change some instruments... sometimes there is a 
poor harmonica for the singer voice and i want tochange it to be played 
by the piano!)
Thanks for your answers!
Please excuse my poor english!