Mine would have to be Edmund after he stops being evil/odd/nasty/a horrid little brat. But then there is also Corin from the horse and his boy, and Puddleglum and Reepicheep... oh man, I love them all!
more minor transformations --
Jadis/The White Witch (although evil transforming to more evil, I don't know if that counts, it's rather more of a progression (regression?) than a transformation)
Shasta/Cor (is slave in rags to the riches of a crown prince transformatiion?)
Trumpkin the skeptic/Trumpkin the believer
dead Aslan/resurrected Aslan
We're all forgetting a certain pretty girl called Susan here. She changed too. Changed into an adult (or so she thinks) who doesn't believe in fantasy worlds anymore. It was sad. It's like CS Lewis is telling us to believe in this joy of fantasy and the joy of realistic fantasy.
I don't know about that. It seems more like he was saying "believe in what you know is true" -- no matter how much the rest of the world has no clue about it. "The seven friends of Narnia" are a pretty small bunch of people to be able to talk about Narnia with.... Susan got carried away with the things and people of her birth world, and didn't have time to think about what happened in that wardrobe during one summer, nor about the brief visit a year later....