I have an award flight in J (advantage rate) for EWR-SIN next January, but I'd like to modify it and go to Bali with a stop over in SIN (ie EWR-SIN-DPS).

There's saver inventory for SIN-DPS, but there is not additional award inventory for EWR-SIN. If I contact SQ, can they cancel my current booking to release the EWR-SIN space and then book the desired EWR-SIN-DPS itinerary? I'm concerned about potential cancellation without the original flight returning to inventory. Any advice would be appreciated.