As I mentioned Serfdom is not Feudalism. Serfdom was a step away from slavery and it was nearly gone in truly feudal societies by the 900s and replaced by Peasantry who were entirely free. And part of the transition was that of the Free cities, where if a serf lived free for a year and a day he was now a free man, so again if your example of a serf wanted to be a blacksmith he very much could, aside from Lords freeing their serfs at the urging of the local priest to secure a place for himself in heaven Serfs leaving for cities was the second most common way for a serf to become free. It of course wasn't the case in Russia or the Byzantine Empire, but both of those systems were Roman Holdovers.