Fighters who became significantly better during their careers...whether it's because of experience..a different trainer..or a different weight division etc.
I remember when I first watched Bernard Hopkins as a young IBF champion.....If you would have told me that this guy would morph into one of the best technicians I've ever seen...
Glen Johnson had 9 losses and was seemingly a journeyman boxer when he beat Eric Harding, Clinton Woods, Roy Jones and Antonio Tarver one right after another. Not only became a unified lightheavyweight champ but was Ring magazine's fighter of the year.
I'm not sure he became better, rather he got better judges and more hometown fights. he has always been a blue-collar workhorse performer.