xAONEx rules clearly state you fly in lower cabin if your class (fare bucket) is not available
Nothing in the rules about compensation.
100's of people have bought A or D and flown in a lower cabin
From rule sheet downloaded a few days ago
ONEWORLD EXPLORER FARES
AA/AY/BA/CX/IB/JC/JL/JO/KA/ LA/LP/MA/NU/QF/RJ/XL/4M
RWR1 RULE 3015
AIR TARIFF RULE 3030/PAT RULE O030
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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.