This notion that a Dom will begin and get trained as a sub is rather alien to much bdsm stories (and movies) isn't it? Not claiming to be much of an expert on bdsm books but in classical books like The Story of O and Venus in Furs, a Dom/me is and was always a Dominant, it's kind of locked in the genetic code, the core of their personality and that helps build their authority. The idea that Sir Stephen had been trained as a slave earlier would pretty much ruin The Story of O.