Shinkansen Reservation

Q1. What happens if I miss my reserved train seat? Will I be issued with a new one?

Q2. What happens if I reserved a seat for the 6pm train but actually decided to catch the 4pm? Do they just cancel the 6pm reservation and reissue another one for the 4pm?

Q3. Am I able to do multiple reservations just incase I miss a train?

What happens if I miss my reserved train seat? Will I be issued with a new one?

With a Japan Rail Pass:
You should make an effort to avoid this scenario. If it happens, nevertheless, you should cancel your old reservation and make a new one.

Without a Japan Rail Pass:
You can ride in an unreserved car of a later train along the same route.

What happens if I reserved a seat for the 6pm train but actually decided to catch the 4pm? Do they just cancel the 6pm reservation and reissue another one for the 4pm?

With or without Japan Rail Pass:
You can change the reservations to a later train at the station of departure.

Am I able to do multiple reservations just incase I miss a train?

With Japan Rail Pass:
You could theoretically do so, but it would be an egoistic action, that, if done by too many other pass holders, would likely result in the discontinuation of free seat reservations at some point in the future. Only make reservations if you are actually going to use them!

Without Japan Rail Pass:
You would have to purchase multiple limited express tickets, which is a waste of money.