Originally Posted by
SJUAMMF
You don't really have to worry about the reserve seat you have on any particular train. Once you get to the Shinkansen Stn, be it Tokyo or Shinagawa, and there is an earlier train available; just make a change at a window or counter to the earlier train. JR don't charge anything for the change.
They don't charge for the 1st change. For the 2nd change, you have to cancel and refund first. It costs up to 30% for the express ticket and 210 yen for the fare ticket. You might not need to refund the fare ticket.