Some of the boys change more than others. Jack changes substantially from a head choirboy - " chapter chorister and head boy" - to a savage leader which supports the statement about Simon in particular as he has a 'natural' goodness which flows from his connectedness to nature. He does not necessarily do the right thing because of any moral code (like Ralph does) instilled in him but just because it's the right thing to do.
Ralph's vision of how things are is all-too-human and clouded compared to Simon's.
It is partly Jack's recognition of Simon's inner strength that threatens Jack and
when Simon seeks to expose the beast as just a "dead man on a hill," he is killed by Jack's group.
Piggy does remain consistent - even his hair doesn't seem to grow. He worries increasingly about what grown-ups would think and what his "aunty" would do. He warns Ralph about the foolishness of even tasting the "meat" that Ralph begins to crave but which means corssing paths with Jack. He has reognized Jack's inherent cruel nature and knows he would not be safe alone with him.
Piggy is doomed in a society where irrational fears and physical strength are more respected than science, law, and dialogue.
Ralph loses his innocence despite trying to keep to the moral code he has learnt from his father (the navy commander). Ralph recognizes Piggy's contribution and knows things are irretrievably changed after
the fall through the air of the true, wise friend called Piggy.
Had the boys not been rescued, it is feasibe to assume that Ralph would have done everything necessary to stay alive. His imperfect nature
....contributes to the gradual descent of the boys into a savagery to which Ralph himself succumbs.
Roger develops his sadistic side and the twins 'Samneric' are eventually manipulated by Jack and join his tribe. Only Simon and Piggy remain true to their original ideas and beliefs. It is significant that they are both killed by the boys.