I have always thought of it as a compliment to be considered a "workout warrior." As such, it most assuredly doesn't mean a player is bereft of the required talent and abilities to to be labeled as a success. The fact that he found more early success as a safety doesn't necessarily mean his skills as a CB wouldn't have eventually proven successful there. If he'd been afforded yet another year to develop at that considerably more demanding position, I think it's conceivable that his development in year two might have done great favors for his reputation.
Moving Jones around constantly throughout his rookie year did precious little to aid his cause in making a favorable impression and, in fact, did exactly the opposite. Dallas has long been prone to moving players around to make up for their frequent lack of depth at crucial positions. In any event, Byron should prove invaluable as a FS and a sure-fired improvement over anyone we've recently employed there in coverage.