From my experience.. this is a task for http://www.airlinecustomeradvocate.com.auLodge a complaint and provide all of your correspondence from Qantas Airways... Sadly, this seems to be the only way to reason with Qantas and partially their Qantas Frequent Flyer service centre.. all the best..

This is a known issue.. as mentioned due to the "change of gauge" ie. change of aircraft using same flight number.. the only way to pre allocate seating is to call QF reservations.. and they will assist... mentioning "change of gauge" may help...