I've said this many times before, but it's only the reservation that's important. If the reservation is on AA then that's where your miles go. The ticket is valid on any flight on any oneworld carrier that flies the segment in question and is operated by a oneworld carrier. The tickets are NOT endorsed with any restrictions on flight date or time and are explicitly endorsed that they are valid on any carrier. You don't need to change the ticket to change to a codeshare, you just need to change the reservation.