You can call SQ and add the F saver waitlist to your current J advantage awards - do not cancel because you may lose even your J advantage seat, as there's no guarantee that the cancelled J advantage immediately are returned to the award pool.

Should you monitor that the F saver subsequently become available and your waitlist isn't cleared (this can happen, due to the disconnect between the SQ availability system against clearing of waitlists) - again you should call to have them secure the F saver first, then cancel and use your returned miles to rebook.