Flight cancelled, then they rebooked me on Qantas and downgraded me

Business class seat booked on Jetstar to Bali tomorrow. They SMS'd flight cancelled due to "Engineering requirements."The helpful Chat person offered a later flight. Is it still Business" I enquired. "Yes" he said. Only told me the flight times, not airline. Turns out to be Qantas and now I'm in Economy. Qantas. not unreasonably, said they can't change the booking as it wasn't made through them. Jetstar says they can't change a Qantas booking (They apparently could make the booking initially). And no refund can be made.