I believe changing carriers would require a ticket re-issuance, however I doubt it would be enforced between BA and QF on a SYD-LHR flight where they code share on all flights. My guess however is booking the QF code share number for the BA flight or booking the BA flight itself when you hold a QF flight number ticket would result in the same outcome and not be any easier either way.
I would chose however to book the QF code share to avoid any problems at the airport.
rich