Henderson (22-5) was originally scheduled to headline the card against Thiago Alves (21-10), but Alves was forced off the card because of a rib injury. Masvidal (29-9) was already scheduled to fight on the card and now slides into the main event.
The UFC officially announced the rumored switch on Saturday. The event, which takes place at Olympic Gymnastics Arena, will mark the promotion's first in South Korea.
A former lightweight champion, Henderson, 31, temporarily moved to welterweight for a short-notice fight against Brandon Thatch in February. He intended to move back to lightweight, but eventually opted to remain at 170 pounds for the Alves fight.
Masvidal is also a former longtime lightweight. He is coming off a first-round knockout against Cezar Ferreira in a welterweight matchup in July. He trains at American Top Team, alongside Alves.
The UFC has not yet announced a plan for Dong Hyun Kim, who was scheduled to fight Masvidal.