S1 is a Fender version of a push-push where the centre of the knob moves, if I remember correctly. I don't see why you couldn't replace it with a push-pull (e.g., CTS) as long as the shaft type and pot value are correct for your guitar. It just switches between parallel and series connections between your pickups. If I remember correctly (from what I know of Jazz basses with S1) the neck becomes the master when this switch is engaged.