We are afraid that you can't change the due date of payments.
When you apply for "Omatome Seikyu Service (combined billing method)," the due date follows that of the main line (if there is a docomo line, the due date follows that of docomo charges).
Therefore the due date of the sub-lines may change.
*Click "here" for the details and application for "Omatome Seikyu Service."