One of the most famous transformations (one that you probably already know about) is the transformation of Matthew Lewis, the actor who played big-eared, buck-toothed, shy Neville Longbottom. We all remember that in his younger years, he was chubby and cute only in a little kid kind of way. Now, however, you can find Matthew Lewis modeling (half-naked at that) for Attitude magazine.
Another incredible transformation was that of actor Harry Melling, who played Harry Potter's fat and bully of a cousin Dudley Dursley. These days, the only thing that is big about Harry is his hair. His frame has slimmed down considerably. Harry has gone on to write a play.
Lastly, another interesting transformation is that of Bonnie Wright, the actress who played Ron Weasley's little sister and Harry Potter's love interest. In the movies, she looks pretty plain jane until we reach the end of the series. We see Bonnie go from looking like a little girl to becoming a beautiful woman.