While a storied part of television tradition, the network late-night shows find themselves with much more competition now with cable programs like "Adult Swim," smaller talk shows hosted by the Comedy Central duo of Stewart and Colbert, Chelsea Handler and a device - a large number of people take that time to watch programs they had taped earlier on their DVRs.
NBC is worried that ABC's Kimmel will establish himself as a go-to late night performer for a younger generation if the network doesn't move swiftly to install Fallon. ABC moved Kimmel's time slot to directly compete with Leno earlier this year.
But the move also has the potential to backfire with Leno's fans, who did not embrace O'Brien when Leno was temporarily moved to prime time a few years ago.
"The guys at NBC are not totally stupid and are not going to shoot themselves in the foot," said Gary Carr, senior vice president and executive director of national broadcast for the ad buying firm TargetCast. "I think it's a good move for them long-term. But it may have short-term ramifications."
NBC has long prided itself on smooth transitions but that reputation took a hit with the short-lived and ill-fated move of O'Brien to "Tonight" and Leno to prime-time a few years ago. In morning television, the "Today" show has taken a ratings nose dive in large measure because of anger at how Ann Curry was treated when she was ousted last year as Matt Lauer's co-host.
The Leno-Fallon changeover didn't begin smoothly. Leno had been cracking jokes about NBC's prime-time futility, angering NBC entertainment chief Robert Greenblatt, who sent a note to Leno telling him to cool it. That only made Leno go after NBC management much harder.
The first public effort toward making the transition smooth came Monday night, when Leno and Fallon appeared in a comic video making fun of the late-night rumors. It aired in between each man's show.
John Dawson, general manager for five NBC affiliates that have extensive reach throughout Kansas, said it will be difficult to give up a program that wins its time period by 33 percent.
"Jay has always been a great friend to the affiliates," he said. "For that alone it will be hard to give up."
But he said he believes in Fallon and in NBC's corporate owners Comcast, the nation's largest cable company.
"Comcast certainly knows how to launch entertainment programming," Dawson said.