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Mr. Leno, who briefly moved to prime time to make room for MR. O'Brien, has proved himself a wily survivor. Despite the challenge from MR. Kimmel, he and “Tonight” have remained top-rated so far.

But if a change is destined, NBC could be eying next year’s Winter Olympics in Sochi, Russia, as a premium launch pad for a revamped “Tonight,”Mr. Adgate, the analyst, said.

“If they’re going to do it, it makes sense to do it when NBC is dominating television for 17 days,” he said, and can heavily promote the “Tonight” switch. “They could have Fallon in Sochi, talking it up.”

In the April issue of GQ magazine, “Late Night” producer Lorne Michaels (“Saturday Night Live”) said a transition to Mr. Fallon had “an inevitability to it,” adding that he was the closest to Carson that “I’ve seen of this generation.”