Well it appears Qantas are using this rule against you
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NOTE: For flights where First or Business Class is not offered or available, passengers may travel in a lower Class, in the applicable booking code for that lower Class. The fare for the highest Class used applies, without compensation.
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However it's now up to you to throw that rule back at them as it's very gray. I interpret that as when booking the ticket, if a particular flight doesn't offer a certain class, it's my own choice to accept a lower class. But it doesn't mention what happens if there is an aircraft change like what you had. The rule says "not offered or available" - well when you booked it obviously was offered and available and QF should really offer a part refund.