A few terms come to mind when describing the character of Yeh Lut Chai from ROCH: dutiful, responsible, moral, loyal, honest, courageous, humble. The man was kind of like a poor man's version of his father in law, Gwok Jing.
"Ambitious," however, is one word that would not describe Yeh Lut Chai; he was never the power- or prestige-hungry type.
So where did he suddenly find the ambition to become Beggar's Union Chief towards the end of ROCH?
I'm not surprised that Yeh Lut Chai took up the challenge of the martial arts contest. He was a consummate martial artist and enjoyed a good challenge. He did know, however, that if he won, he would need to assume the role of Beggar's Union Chief - a prestigious title that entailed a great amount of power and responsibility.
It doesn't seem in character for Yeh Lut Chai to pursue that, especially since the office was open only because his good friend Lo Yau Gerk, the previous Union Chief, had been murdered recently.