I had this happen to me last year flying business BNE-MNL via SYD. The evening before my flight, someone from Qantas called me to advise that my BNE-SYD flight had been cancelled, and if I still wanted to fly business class on another BNE-SYD flight I would have to catch this particular one, which I agreed to.

So next morning I check in, go into the lounge at BNE and only realise upon boarding that I've actually been placed in the first row of economy, which pisses me off. I had overheard others saying it was a sold out flight, I'm guessing they oversold business and then bumped me out or something on purpose. They tried to call me while I was in MNL after I complained on their Facebook page, but I didn't have roaming. Upon returning to Australia I saw that they had credited 10,000 points to my account.