Oh sorry - I misunderstood. I thought you meant that you had changed your reservation on the same route to a different day. Obviously if start and end points on the next coupon on the ticket doesn't match the next flight you want to take then you have to get the ticket reissued. If BA or QF or AY does it there will be a fee for doing any work in addition to the change fee; no-one has found a way round this. If CX or AA does it you pay only the change fee.