Can a partially flown Oneworld Explorer purchased from an airline (AA) be modified by a TA to replace airline sectors with codeshares (e.g. QF) consistent with Oneworld Explorer rules? While legal in Oneworld, AA have a policy of not using codeshares on AA metal.

I appreciate there may be an increase in taxes/YQ which would have to be paid as a result.

Can this be done, or is the booking locked to AA, and only AA can make changes?