NEW YORK – Jimmy Fallon will officially be given the keys to NBC’s “Late Night” franchise following Conan O’Brien’s exit.

A person close to the negotiations who spoke on condition of anonymity because the announcement hadn’t been made confirmed the widely rumored change Sunday and said a news conference was planned for Monday.

All that’s left is an official date for NBC’s transition: O’Brien moving out West to take over for Jay Leno on the “Tonight” show and Fallon following in the next time slot.

Fallon was a “Saturday Night Live” cast member before leaving for a movie career in 2004. He has had mixed success, with his most notable role as a childlike Boston Red Sox fan and Drew Barrymore’s paramour in “Fever Pitch.”

He signed a deal with NBC in 2007 that was widely seen as a way to keep him in the fold until the time came for Monday’s announcement.

It would be hard for him to have a rockier transition than O’Brien had when he took over from David Letterman in the mid-1990s. O’Brien was a writer with a quick sense of humor but a mystery to the television audience, and he was savaged in early reviews. NBC nearly fired him but persevered, and O’Brien grew into the role.

It was in large part to keep O’Brien happy that NBC announced four years ago that he would replace Leno next year.

While Leno went along with the plan, there are reports that the workaholic comic is not eager to leave. NBC Universal is trying to find a job that would satisfy him and avoid having Leno move to ABC or Fox to compete directly against O’Brien.

NBC could decide to back out of the plan and keep Leno on “Tonight,” but O’Brien’s contract calls for a penalty fee reportedly close to $40 million.

It all amounts to a roll of the dice in late-night, where NBC has stayed on top despite the network’s prime-time problems.

NBC’s announcement opens network television’s “upfront” week, when fall schedules are set and presented to advertisers. NBC took the unusual step of announcing its schedule a month ago, and it has invited advertisers for a Monday sales presentation emphasizing all of NBC Universal’s properties.