We currently have a gas range cook top with electric oven underneath that is hooked up to a 40 amp breaker via 8/3 romex wire. We are replacing this with an all gas range/oven that only requires 120 volts/15 amp breaker. My question is can I use the existing 8/3 wire by splicing 12/2 via one junction box at the service panel end and another box in an accessible spot just before the new oven, capping off the red wire at both ends? I'd then replace the 40 amp breaker with a 15 amp and connect 12/2, then connect the other spliced 12/2 into a new receptacle for the oven.
My original plan was just to attach fishing wire to the existing 8/3, pull out the 8/3 from the service panel, then attach 12/2 to the fishing wire and fish that through to the service panel. I will go this route if I have to but it involves several cuts into drywall, undoing bulkheads and acoustic tiles, etc.