Transformation and metamorphosis are common in The Voyage of the Dawn Treader. Although some transformations seem purely physical (such as when Eustace turns into a dragon), affecting only the outward form and shape of a person or people, they usually turn out to be related to emotional, psychological, or spiritual changes as well. Transformation is not just imposed by external forces; instead, it reveals a deeper truth about the person or people who undergo it. Transformations that seem unpleasant or distressing often turn out to serve a greater purpose in the end.
Eustace's transformation into a dragon is merely a physical confirmation of something that has already happened to him psychologically and spiritually.
Eustace's most important transformation is his realization that he does not have the power to change himself. He must want to change, then submit to Aslan, who can actually effect the change.