He is changed, because before the Journey, he was arrogant, power-hungry, and also, he destroyed the grand temple. So, the Buddha trapped him under a mountain for 500 years. Later Tang Seng, a monk, rescued the monkey from the mountain by pulling off the talisman that made the monkey unable to escape, so monkey was very grateful.
He then wants to protect Tang Seng as much as possible(because he is grateful), and along the way, they face many monsters. The monsters have heard that if they eat Tang Seng, they will be immortal. Some of the monsters they encounter are very smart. They disguise themselves as humans, and only monkey can see through their disguise, the others cannot (oh, and by the way, he also makes 2 friends that were former monsters), so to Tang Seng, it looks like monkey has killed an innocent person, and then Tang Seng says the tightening crown spell that makes monkey's head hurt like crazy, then makes him go away.
He then comes back when he senses Tang Seng is captured, and this thing happens again, and again for three times, until Bodhisattva(Goddess of Mercy/Guanyin) tells Tang Seng that only monkey can see through the disguises and convinces Tang Seng not to say it anymore, and monkey is very grateful. Once they are back from the journey, monkey has changed. He becomes responsible, honorable, and then he stays with Tang Seng, and he is not as ruthless and naughty.
