The best place to ask is BA and get it to rebook you onto another QF flight. If booked in J class , it should be reasonably easy to find availability on a dofferent service

With a BA ticket, I wouldn't expect a lot of chance of getting QF to do it at the airport on the day and more likely that if you do go early to the airport that you will wait around in Brisbane - but get there a couple off hours before departure and see whether the agent will put you on the previous service to one booked to Sydney