Basic zoology 101: Snakes shed their skin in order to grow. They get slightly bigger, they look slightly shinier and sometimes they even change colors a bit. Inherently though. They still stay a snake.
Basic Entomology 101: Caterpillars can and do change into butterflies. They grow wings, they lose tons of weight (human sized cocoons would be a international weight loss craze) and they become beautiful living mosiacs. Inherently, they are not the same.
Basic redemption 101: am I a snake or a butterfly? Can the snake choose to become something else? Can the Caterpillar stay the same?