My 2 are:-
1) Hector Camacho (the original) - He was such an exciting & attacking fighter, then one night, Edwin Rosario stunned him to the sole of his boots & from that point on, he became one of the dullest & most safety conscious fighters ever. It was even more remarkable because the transformation happened during a fight & he never really reverted back to his original style after!!!
2) Donovan 'Razor' Ruddock - He was more of a boxer to start with, then when he sparked Michael Dokes with his vaunted 'smash' punch, he morphed into a one dimensional slugger who thought he could KO every opponent with his honey punch! :P