Yes, it is possible to change your seat assignment.
To change your seat assignment, log in to the Check reservation page shown below and complete the necessary procedures.
「Check reservation」（Login page）
*You will need your reservation number in order to log in.
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*Caution* In order to change your seat assignment, it is necessary to pay the seat assignment fee. When you request a new seat assignment, the seat assignment fee you have already paid is not refundable. Seat assignments can be made up to 5 hours prior to the departure time. |
Reference What is a reservation number?