I can give examples of great rationalists who never admitted to being wrong in particular debates. Some defenders of the phlogiston theory whose names I can't remember. Einstein. Numerous famous biologists in their attacks on sociobiology. Other famous biologists in their attacks on group selection. I'm pretty sure you could come up with a nonending stream of examples if you studied the history of science.

There are a few people here on LessWrong whom I think are great rationalists, in an absolute sense; but who AFAIK have never acknowledged being caught in a mistake. That indicates something wrong. Even if you really never have made a mistake, that would indicate that you haven't tried anything hard.

(P.S. - You get no credit for changing your mind by realizing your error yourself, partial credit for changing your mind after reading something written by a non-threatening dead person, and full credit for admitting to someone during an argument that they were right and you were wrong.)