Whether you can board an earlier Shinkansen train depends strictly on the type of ticket you hold. If you have a Standard Reserved Seat ticket, you are generally allowed to board an earlier train on the same day, but you must sit in the Non-Reserved (Jidoseki) cars; your specific seat reservation is only valid for the train number printed on your ticket. If you wish to have a reserved seat on the earlier train, you must visit a ticket office (Midori-no-Madoguchi) or use a ticket machine to change your reservation before boarding. However, if you hold a Discounted Ticket (such as the "Hayatoku" early-bird fares or certain tour packages), these are often strictly bound to a specific train; boarding an earlier one may invalidate the ticket or require you to pay the full fare difference. For those using a JR Pass, you can simply visit a kiosk or counter to change your reservation to an earlier time for free, provided seats are available. If you accidentally board an earlier train without changing your ticket, you should find the conductor immediately, though you will likely be asked to move to a non-reserved section.