On the SX700, there is a flashing dot that indicates if a channel is playing notes (the dot is to the left side of each channel’s volume). I guess there is something similar on the S970. If the dot is not flashing for a certain channel, it means that there is no data in it so it does not make sense as such to change the instrument of the channel. You will then need to proceed as Derek has suggested, or simply use another style where the pad channel is actively used.