In his 18th fight, and last before challenging for his first world title, Chang scored a 10-round unanimous decision over former WBC light-flyweight champion Amado Ursua.
However, in a rematch held in Seoul, he avenged that loss and became WBC light-flyweight world champion by knocking out Zapata at 2:46 of the 3rd round.
Chang then went on to establish a then-world record for the most defenses as world light flyweight champion, defending the title successfully 16 times between 1983 and 1988 when he retired.
After securing victories over Efren Pinto and Agustin Garcia in 1987, he faced future champion Isidro Perez.
This is why Chang Jung-koo became financially difficult despite winning 15 championships, which forced him to return to active duty in 1989.
However, financial difficulties pushed him back into the ring in 1989 when he challenged and lost to Humberto González, who was the WBC light-flyweight champion at the time.
Chitalada then lost that title to Muangchai Kittikasem, prompting Chang to challenge the newly crowned champion.