If you need to add to the itinerary, you can keep your existing legs and record locator and just pay the 3100 yen cancellation fee and just add to it. At first the JAL agent will tell you they have to cancel and start over. Tell them the seats you have are no longer available. They will check with a supervisor and come back and say they can in fact keep your existing seats. I've had this happen four times now. Same song and dance each time.