Actually, Mikey if the midis are already on the computer he could just as easily change sounds using just about any sequencing program, like Power Tracks, Cakewalk, Cubase, etc. I'm sure he could find something for free on the internet or maybe even got something with the Casio.
As for USB, he only needs to connect the keyboard to the computer. He should be able to see his connection in the Windows Explorer screen or maybe Casio has some connectivity software to facilitate this. Anyway, simply transfer the song(s) to the Casio. Make the changes, then SAVE the file to seal in the changes. Make sure that the file extension stays as .mid not a Casio specialized extension. Transfer the song(s) back to the PC. Done.
[This message has been edited by cassp (edited 01-09-2008).]