Due to holiday season, we’d recommend you to pre-book your seats upon arrival in Japan.
You don’t have anything to worry about though, as the trains rarely are full. But it can still be a good idea to book a seat beforehand to be 100% sure. It doesn’t cost you anything to do, so better safe than sorry in the end.
Your seats for the entire trip including changes can be booked and routed by the JR staff at your nearest JR Office. They can be found is any larger airport or train station.
Wish you the best stay in Japan and let us know if there’s anything else we can do to help.
With kind regards, Jonathan H.