OK can I check I understand the issue properly?! You were booked on a QF flight that is now cancelled. I don't know QF's seating policy - were you able to choose seats for this flight? QF should offer you a refund or a reroute, i.e. the new BA flight.

BA's seating policy will now apply. BA will charge you £60 to pick seats if you want to do so before check-in (more expensive for the upper deck I think?). If you are oneworld sapphire = QF Gold you get free selection. If you do a dummy booking on expedia you can preview the seat map. Most would advise the upper deck of the 747, row 62 or 64.

If you've gone from a flight with free selection to now having to pay, I guess you'll be annoyed. But I'm not sure what you can do except make a fuss or insist on a refund. There's no reason BA should give you free selection - it's QF's problem and they'll have to pay/request it for you if you can convince them they should.