Being asked to pay to move your flight forward after a Qantas cancellation is outrageous, especially when it involves a downgrade. I would absolutely be submitting a complaint to Qantas customer care on the basis of that treatment.
Yes, usually when IRROPS happens and/or there's a flight cancellation, on revenue fares people are moved without penalty, whatever fare buckets available. Another one notched down to call centre incompetence?