In 2016 on a SYD-JNB flight I too was downgraded by Qantas. Your experience was better than mine though OP, the check-in agent at the Business class counter didn't even bother to inform us. I only realised we were in Economy when I looked at our boarding passes. When raised at the customer service desk we were told (by a supervisor who I still vividly remember as appearing to be 'under the influence') that there would be no compensation and our choice was either to fly in Economy or not fly at all.
To make matters worse, on the return leg due to some catering complications they
did not load any food. Managed to get one beer from the leftovers still on board from the previous flight and that was it for the twelve hour journey. Thankfully managed to have a meal in the JNB lounge but the promised compensation and meal vouchers on arrival in Sydney failed to materialise.
Safe to say this was my last experience with Qantas, I have shifted my business and become a passionate supporter of
Virgin Australia. Currently in the top 10% of their frequent flyers with 20+ trips annually on VA and their partners, never had a bad experience and never looked back. QF's loss. Taking your money elsewhere is the only way they will learn.