How do Roger and Simon change from the beginning of the book to the end?
This is a difficult question. The reason for that difficulty is that I do not believe that Simon and Roger changed much throughout the book, if at all. Roger starts out as evil and sadistic and stays that way. If anything he just gets more awful. In chapter 4 Roger and Maurice kick over and destroy the littluns' sandcastles. No reason for it. Just to be mean. Then he starts throwing rocks at other kids. By the end of the novel, Roger just graduates to bigger rocks. "With a sense of delirious abandonment" he pushes the rock that hits and kills Piggy. The boy is sick.
Simon, on the other hand is the complete opposite of Roger. Simon starts off as the moral compass of the group and stays the moral compass of the group. While the other boys struggle to stay moral like society has taught them, Simon has no such struggles. In fact, his clarity of values is what allows him to understand that the evil (Lord of the Flies) is not some external monster, but the potential that each boy has for that evil. After that realization Simon is eager to prove to the boys what he knows. An act of altruism. Unfortunately he is killed when Jack, Roger, and the rest of the boys mistake him for a monster of the jungle.
You can find more information about Roger and Simon's change in the links included below.