I think once you have been truely knocked out the memory (and fear) is always there. Deep in the subconscious, you know that you are vulnurable, and I think the mindset of the fighter changes. This coupled with getting older, I think some of that edge is lost. And I do agree the body installs a "safety switch" that prevents future damage.

If a person has never been knocked out, it's a strange response for the body. The body dosent know what to do. After several times, it may become routine.