Both John and Lorraine change a great deal over the course of the book. What is great about this book is that the chapters alternate between those two characters telling the story, so readers get really solid insight into how each character is mentally and emotionally developing throughout the story.
When the story begins, readers see John as a mischievous troublemaker of a kid. He does not care what other people think about him, and he has a huge problem with authority. That is why he sets off firecrackers in the bathroom and antagonizes substitute teachers. I believe that John's attitude and actions result from the fact that the important adults in his life don't care about him. John's dad thinks it is hilarious that his son is going to grow up to be an alcoholic. By the end of the story, John has changed a great deal. Through his relationship with the Pigman, John has learned what it feels like to be loved. He has also learned what it feels like to hurt and disappoint an important adult in his life. John feels horrible for the party he hosted and the resulting sadness it caused the Pigman. Losing the Pigman also changed John. It really showed him that relationships are important and have to be cherished and cared for.
In some ways, Lorraine learned the same lessons as John, but Lorraine's changes are also distinct from John's changes. Lorraine's home life is equally as messed up as John's home life. Her mother is equally abusive, but in a different way. Lorraine's mother is constantly criticizing Lorraine, and that results in Lorraine being very shy and having almost no self-confidence. She has hopes and dreams, but she doesn't really have enough confidence to pursue those dreams; however, by the end of the book, she has learned from John how to be a bit more confident in herself. When the book ends, readers feel that her dream of someday becoming a writer just might come true. This is bolstered by the fact that she and John are the ones "writing" the book we have been reading.