If travel has not started , then a reissue can cause a complete rebooking with new pricing and new rules

Once travel has commenced on an OW ATW ticket, then it can be changed in accordance with the rules in force at the time of issuance. This is regardless of whether rule changes to newly issued tickets have been changed benefically or banefully. e.g. if change fees / cancellation penalties were to be increased on new tickets, this would not affect someone whose journey was already underway

If MX was not listed in the valid carriers for the ticket rules when the OP's ticket was issued , since it is already underway, MX will not become a valid carrier now.

I still think that QF's statement that MX change is invalid is correct, but that staff member's explanation of why is flawed