Chage SMF voices on various tracks on the S900 and save it within a SMF on the Yamaha ?

I don't usually use SMF's but I'm playing a midifile of Larry Carlton's Room 335 and playing the guitar part with my AX7 Keytar, the drum track keeps reverting back to a grand piano and I have to change it every time. its driving me crazy!!! tried to do it logically but that did'nt work? there must be a easy solution?

Need you're help, all you experts out there..

Rgds

Noel J