Not true, at least by my understanding of what constitutes a "theory". Theories invoke some well developed set of causes, principles or mechanisms to explain some relevant set of scientific facts or laws. To my mind this is the key characteristic of theories, that they explain why the facts are as they are, rather than some way they might otherwise have been. (This is why good theories must be falsifiable, btw. The power to explain or account for some set of facts entails that the theory, if it be true, must prohibit the instantiation of many other sets of facts.)
Therefore, no matter how detailed or profound our understanding of "how it works" may become, evolution will always remain a theory simply because it is an explanatory principle used to account for numerous facts and phenomena in biology and related fields. Theories may be modified or falsified and abandoned, and they be more or less controversial, and they may be held with varying degrees of confidence, but they do not "graduate" or "advance" to being anything other than theories.
Indeed, quite the opposite of what you suggest, the more a theory says about "how it works," the more "theory-like" it becomes.
As I recall it from my college days, an idea "graduates" from a hypothesis, to a Theory, to a "Law", as more experimental and observational data is compiled.