If you want to travel in a Green Car, you will have to top it with a Green Car reserved seat ticket fee. And if you plan traveling in a Premium Green Car, the price consists of the basic fare, the limited express tickets, and the Premium Green reserved seat ticket fee. Yes, seems like a lot to take in, but it's very simple once you grasp the idea.
The same rules are true for Shinkansen. Traveling in an Ordinary Car means paying a basic fare as well as for a super express train ticket (the second class offers both reserved and unreserved seats). First-class Shinkansen ticket = basic fare + super express ticket + Green Car reserved seat fare. A Gran Class ticket cost is made of almost the same components, but you will need to pay for a Gran Class reserved ticket instead of a Green Car one. See? Easy-peasy!
Reading Train Tickets in Japan
Buying Train Tickets in Japan
Q&A: Japanese Rail Tickets
Yes, it's possible to exchange a Shinkansen ticket, given that it is unused and still valid. Although you won't get the full price back, as there is a cancellation fee paid for every exchange operation.