And bear in mind:
When you add the return sector it needs to be reticketed, and this can take months, and you cant assign seats on the return leg until this is done.
As others have said, use the Japan call centre. The USA one refused to do it, saying I had to wait until the UK call centre opened. Only when I called back and pleaded my case (that for CPT the seats would have gone by then) did he reluctantly agree to do it after speaking with a supervisor and as an exception.