the rule is (or was, as far as a few months ago)
1. prior to departure, free re-route. (7 days prior if from TC1, i.e. NA/SA)
2. after departure (i.e. >1 coupon used), free is date/carrier change; $75 if re-route. so sure if ex-Oz is specially treated
AA has been a pain at insisting changing carrier will be charged (but they are wrong).
i.e. same routing, JFK-LHR, or HKG-LHR but switching carrier. BA/CX always take my ticket without any questions (and they are repsecting the OW contract), but almost all people in the RTW desk have a different (and wrong) interpretation.