I believe they are right as a policy, but "can't" is not true...there's always a way. I successfully changed HKG-SIN-LHR-OSL to HKG-SIN-LHR-CPH after ticketing. At first they refused but I said it was the same zone/cost and it had been done before. They put me on hold and said they'd make an exception for me as a Gold and went and did it ^ Very pleased as it allowed me to make an important meeting immediately after my holiday and there was no other flight I could have taken. Lucky all round!