I am currently in the middle of a DONE3 and have a flight I am trying to change. It is a free change as per the fare rules.

The ticket was issued by CX. The marketing carrier of the flight in question (LAX-IAH) is AA, operated by US Express.

The flight I am trying to change had a flight number change prior to ticketing which wasn't auto updated in the booking, and was then ticketed with the old flight number. I asked AA about that at JFK (after the first 2 sectors had been flown) and after looking at the booking, saw that the flight number was wrong, but couldn't fix it. Said to go to CX.
I contacted CX US, they said "We don't operate LAX-IAH" and refused to deal with it. Contacted CX UK, they fixed the flight number with a note "Our system is unable to revalidate your ticket but the flight is confirmed and US Airways will need to revalidate your ticket on departure."

This flight is the next sector on the DONE3. I'd like to change it to the later flight. I went to AA as the marketing carrier. They said to contact US as the operating carrier. When I spoke to US, the agent at first couldn't find a valid e-ticket attached to the PNR (I have flown 3 AA sectors on that ticket since CX UK fiddled with it) and contacted the rates desk who claimed it was an award and not a revenue ticket. When I told them that is wasn't an award, they contacted rates again and looked at it closer and have now said the rules in their system say to go talk to CX.

This is the first time in 8 years of xONEx tickets where I've gotten sent to the issuing carrier by the operating carrier to make changes after the trip has started. (I had a CX issued DONE3 last year which had AA, QF, BA, MH, JL and AY all make changes at different points). The oneworld website even says "If you need to make changes to your itinerary during your journey, please contact the member airline you are flying with on your next segment." so I have no idea why US can't make the change.