"Transformation" sounds all optimistic and happy (think butterflies!) but, in fact, it can be a terrible thing. Take Dorian Gray, for example. He begins in The Picture of Dorian Gray as a kind of human butterfly – gorgeous, innocent, pure, perhaps even pleasantly mindless. However, he then undergoes a dramatic transformation that renders him grotesquely un-beautiful (on the inside, at least)…the splendid butterfly eventually turns into an icky caterpillar. Um, no offense, caterpillars – we think you're awesome too. It's just a metaphor.
The static nature of the other characters (primarily Lord Henry and Basil) serves to highlight Dorian's dramatic evolution over the course of the novel.
Transformation here is an inexorable process – once it begins, there is no turning back.