NBC has been desperate to avoid a repeat of the circus-like atmosphere that accompanied previous transitions on "Tonight," the executives said. The changing of the guard is one of the biggest personnel decisions in television, and has always been fraught with intrigue and back-room maneuvering.
Three years ago, NBC's effort to replace Mr. Leno with Conan O'Brien ended in recriminations and a definitive reversal; Mr. Leno was reinstated as host after only seven months, and NBC endured weeks of negative news coverage. In the early 1990s, Mr. Leno and David Letterman engaged in an often acrimonious competition to replace Mr. Carson.
But a transition totally free of tumult may be difficult to accomplish. Already there has been sniping between Mr. Leno and NBC's top entertainment executive, Robert Greenblatt, over some stinging jokes Mr. Leno made in his monologue about the failure of NBC's prime-time schedule.
Mr. Greenblatt, who is responsible for that schedule, directed some pointed criticism at Mr. Leno in an e-mail to him. Mr. Leno stood his ground in a response, asserting that jokes spoofing the network are part of the job for a late-night host.
Mr. Leno, who will turn 63 next month, has continued to take shots at NBC's management. On Monday, he joked about how the snakes that St. Patrick drove from Ireland came to the United States and became NBC executives.
The network has sought to temper the feud while it works out its plans for the show's future—which center on Mr. Fallon.
Mr. Leno's "Tonight" show still regularly leads in the late-night ratings. But by turning to Mr. Fallon, NBC hopes to counter what it regards as its biggest late-night competitor of the future, Jimmy Kimmel, who in January moved his show on ABC from midnight to 11:35 p.m.
Many TV executives speculated that NBC could not afford to wait too long to promote Mr. Fallon, or it might risk having Mr. Kimmel, 45, lock up the young-adult viewers who are the economic lifeblood of late-night television.
NBC has quietly begun work on a new studio in its headquarters building at 30 Rockefeller Plaza as the home for the new "Tonight Show," two executives said. The studio is part of a general reconstruction of the building being undertaken by Comcast, which this week completed a full takeover of NBCUniversal. [Comcast is the owner of NBC Universal, the parent company of CNBC and CNBC.com.]
An NBC spokeswoman declined to comment on the move, other than to say the network was building a new state-of-the-art studio for Mr. Fallon.
There are many reasons that Mr. Fallon would prefer to remain in New York, where he has starred on "Late Night" for four years. He is a native of New York State (from Saugerties, just south of Albany) and his longtime association with the producer Lorne Michaels could be kept intact if he stayed in New York. Mr. Fallon made his reputation at 30 Rockefeller as a star on Mr. Michaels's centerpiece show, "Saturday Night Live."
Perhaps most important, Mr. Fallon could ensure the continued participation of his house band, the Roots, who have been an integral part of the show and whose members are close to Mr. Fallon.
A move to New York would also return "Tonight" to its roots, after an absence of more than four decades. Beginning in 1954, it was broadcast every evening from Manhattan, first from the Hudson Theater with Mr. Allen as host, followed by Jack Paar and Johnny Carson, both of whom worked at 30 Rockefeller. But in 1972, Mr. Carson, looking for easier access to Hollywood guests, as well as a different lifestyle, moved the show permanently to Burbank.
Mr. Fallon now occupies the studio where Mr. Carson was working in the 1960s and early 1970s. His "Late Night" show is broadcast at 12:35 a.m. Eastern time, after Mr. Leno on "Tonight."
A New York "Tonight Show" will join a metropolitan landscape already filled with late-night comedy programs, including "Late Show With David Letterman" on CBS and shows featuring Jon Stewart and Stephen Colbert on Comedy Central. One lingering question is what NBC will do with its "Late Night" franchise, which has always been based in New York.
Mr. O'Brien hosted that program before Mr. Fallon, and it had been speculated before his ascension to "Tonight" that he might try to keep working in New York, where he had thrived. But at the time, NBC insisted "Tonight" had become a Hollywood-centric show and needed to stay in California.
Mr. Fallon quickly impressed NBC's new management under Comcast, and his succession has been widely expected for at least a year. The only question has been when.
The potential timetable for the change—sometime in the next 18 months—has been tied to Mr. Leno's current contract, which ends in fall 2014, as well as the need to sign Mr. Fallon to a new deal.
Another complicating factor has been Mr. Leno's success in the ratings.
In recent weeks, he has continued to finish first—always in the category of total viewers and usually among viewers ages 18 to 49, the most sought-after group for late-night advertisers.
As one of the executives involved in the planning of the shift to Mr. Fallon put it: "And then Jay manages to stay ahead of Kimmel. How often has that guy been underestimated?"