Simon is one of the four primary characters in Lord of the Flies, by William Golding, and he is the most sensitive of the four boys. It is true that, in chapter three of the novel, we see Simon do something we have not seen him do before; however, it is probably better characterized as revelation than a change.
Ralph and Simon have been working on the shelters all day, and Jack finally arrives back from his usual day of hunting. Unfortunately, not enough has been done because they have not had enough help, but Jack has no inclination to help, either. Simon listens to their conversation and is a nominal participant in their discussion. He starts to follow them when they leave but changes his mind after a few steps;
...he turned his back on this and walked into the forest with an air of purpose.
He walks a familiar path toward a familiar place (which is why this is not really a change but, since we have not seen this before, it is a revelation).
He walked with an accustomed tread through the acres of fruit trees, where the least energetic could ﬁnd an easy if unsatisfying meal. Flower and fruit grew together on the same tree and everywhere was the scent of ripeness and the booming of a million bees at pasture.
The littluns stop him, begging Simon to get some fruit for them from the higher branches which they cannot reach, and then he continues his deliberate journey to a place he knows well.
The path is barely discernible, but Simon walks it confidently, indicating his familiarity with the path. When he stops, his actions indicate he does not want anyone to see him.
Simon paused. He looked over his shoulder as Jack had done at the close ways behind him and glanced swiftly round to conﬁrm that he was utterly alone. For a moment his movements were almost furtive. Then he bent down and wormed his way into the center of the mat. The creepers and the bushes were so close that he left his sweat on them and they pulled together behind him. When he was secure in the middle he was in a little cabin screened off from the open space by a few leaves. He squatted down, parted the leaves and looked out into the clearing. Nothing moved but a pair of gaudy butterﬂies that danced round each other in the hot air.
Simon has found a retreat, a place where he can go to be silent and, presumably, to think--or at least to have time to think--without the constant clamor of littluns and others.
In later chapters, this is the place from which Simon emerges and meets the Lord of the Flies. Though Simon does not shirk his duties and helps others when he can, it is clear that he is a boy who needs to have some time alone, and this is where he goes.