How did serfs become the merchants who formed the new middle class?

Answer 1

Some serfs left their manors, with or without permission, and settled in towns and cities. In some cases they were encouraged to do so by monarchs who wanted to make the cities grow. Most of these serfs remained laborers, but a few started businesses and were able to achieve a measure of middle class prosperity.

You should understand that the middle class was not made up of former serfs, however. People of the middle class, who were neither land owners, nor serfs, nor clergy, had been around since ancient times. Many craftsmen, artisans, and business owners lived and worked during the entire Middle Ages. The serfs were not even the only people moving into the middle class, as the younger children of nobility sometimes moved to cities and used the educations they had received to be lawyers, doctors, and other members of wealthy middle class society.