I'm going to suggest Clay- Liston. Before beating Liston, Clay wasn't anything special. He barely got by mediocrities like Doug Jones and Henry Cooper by the skin of his teeth. After Liston, he immediately turned into the ATG that is lauded on this forum.
in reading your post, i actually think you may have a better point than me. your thread is fights that changed a fighter "forever." that fight did change him forever. everything about him changed after that. his demeanor, he was likeable in his second career and he was a lot smarter in the ring after this.