1. When something can't be done online, call, call & call. The online system will charge a change fee. The call center won't.
2. Before you call, do the research and be prepared to tell the CSR exactly the flights you want. It's not worth the cost of the call (which is free) to complain that the change doesn't work unless there's something which does work.
3. You also have the alternative of a full refund to the original form of payment.