Yes, you can change your Shinkansen time, but the rules depend on your ticket type and whether you have already "issued" the physical ticket. In 2026, if you booked through the SmartEX or JR-West Online systems, you can change your reservation as many times as you like for free via the app, provided the train has not departed and you haven't picked up a paper ticket. If you have a standard physical ticket purchased at a station, you are allowed one free change to a different time or date at any "Midori-no-madoguchi" (ticket office). After the first change, further adjustments require a refund and a new purchase, involving a fee (typically 340 to 560 yen). If you miss your reserved train, your ticket is still valid for "Non-Reserved" (unreserved) seats on any later train of the same category on the same day, but you lose the right to a specific assigned seat. Note that "Hayatoku" (early bird) discounted tickets often have stricter "no-change" policies, so always check the fine print on your booking.