Could a serf become a knight or noble?

Well, sometimes, especially to be a noble because to be a knight, the horses and armor was expensive, and most serfs lacked skill, but some serfs did become knights.

Nobles... Well, even if you had lots of money in that time period, you couldn't always be a noble. Nobles were from rich families and they had almost always had been rich, especially very rich nobles, but they could sometimes get high jobs, with the money, probably, and be a noble.

You're welcome! ;)