I've had occasional success with moving to an earlier flight on a RTW ticket with some non ticketing carriers but in the 2 occasions with CX they would only do it if they took control of the ticket. In one case I had other changes that I wanted to make and the time to wait so I had them do it. It took about 90 minutes and ended up making the walk to my flight faster than I would have liked. In the other case I did not have the time to wait nor the inclination to reissue for just one earlier segment. I can also say that it has depended on the airport with a carrier (commonly yes with BA at Europe stations but hard, almost angry no at JNB). In several other cases in which I had a half a day's notice of what I wanted to do I got it done on the phone with either the EP or CK desk in time for the earlier flight.