For an involuntary reroute (which this is), QF has a legal obligation to get you there using any airline for similar routing. They are much happier using QF (and you would be too, QF J is better than CX J these days). So you could insist on the CX flight, or QF will use a seat on any QF flight (even if no award inventory). Note that this is different than a voluntary reroute (e.g. if you were late and missed your original flight), in which case award inventory must exist (and might be created for WP at the discretion of yield management).