What does William mean when he says a man can change his stars after his first joust posing as sir ector?

When William says a man can change his stars after his first joust posing as Sir Ector, he is expressing the idea that individuals have the power to shape their own destinies and alter their circumstances through their actions and choices. In this context, his victory in the joust signifies a transformative moment that allows him to rise above his humble origins and pursue a greater future. It emphasizes themes of ambition, self-determination, and the belief that one's fate is not predetermined.