How does the change in Laurie's clothing on his first day of school signal a change his behavior?

The change in Laurie's clothing is described in the first paragraph of Shirley Jackson's short story, "Charles." It foreshadows the behavioral changes in Laurie, as manifested through the character Charles, whom Laurie invents. 

Here is the paragraph: 

The day my son Laurie started kindergarten he renounced corduroy overalls with bibs and began wearing blue jeans with a belt; I watched him go off the first morning with the older girl next door, seeing clearly that an era of my life was ended, my sweet-voiced nursery-school tot replaced by a longtrousered, swaggering character who forgot to stop at the corner and wave good-bye to me.

Jackson's word choice and description of Laurie's new attire foreshadow the behavioral changes to come. First, she uses the word "renounced." Synonyms for this word include shun, reject, and disown. This shows that Laurie is asserting his independence with his clothing choice. One can infer that his mother made the clothing choices up to that point. Now Laurie is casting aside the clothing he used to wear and choosing blue jeans with a belt, a more grown-up choice. His mother, the narrator, describes him as "swaggering," which suggests Laurie feels more grown up and impressive in his new attire.  

The mother describes watching her sweet-voiced preschooler being replaced by a child who swaggers. He also wears clothing that shows he is leaving behind a part of his childhood. He even forgets to turn around and wave to his mother, showing that he doesn't think he needs her as much as he did previously.