Not to preempt the inevitable sequel (three-quel?) to New York Times reporter Bill Carter’s upcoming latenight book, but if you want an advance preview of the next chapters in that long-running soap opera, pull up a chair.

A few weeks ago I indulged in a trip down memory lane, rehashing columns that identified how NBC’s impossible situation — namely, mollifying both Jay Leno and Conan O’Brien — might unravel. At the time, Carnac the (Occasionally) Magnificent promised another look ahead closer to O’Brien’s TBS premiere on Nov. 8.

So what happens next? Stroking the old crystal ball, the prologue is more exciting than the immediate future.

O’Brien will once again premiere to inflated numbers, which is silly, since his new show, “Conan,” is going to look a lot like his take on “The Tonight Show” — which ultimately looked a lot like his first program. Then again, “The Jay Leno Show” didn’t deviate much from his “Tonight Show,” just as David Letterman’s CBS show made only cosmetic tweaks (including an improved wardrobe) from his “Late Night” stint.

These guys do what they do; only the time and call letters change.

O’Brien’s ratings will inevitably soften, but they should settle at a range that’s comfortable for TBS without stealing much thunder from Leno and Letterman after the initial salvo. If anything, his more direct competition will be Comedy Central’s “The Daily Show”/”The Colbert Report” tandem, although given time they’ll be fine as well.

The longer-term forecast becomes more problematic, at least for the major networks. Much has been made of Leno’s diminished results since he returned to “The Tonight Show,” but that ignores the hard-to-quantify damage all NBC’s boomeranging around has done. It’s actually pretty amazing that Leno so quickly reclaimed first-place status in total viewers — edging Letterman 3.7 million to 3.6 million season to date — and regained a sizable chunk of his audience despite negative publicity, NBC’s continuing 10 o’clock woes and Letterman’s lead-in advantage.

That NBC’s younger demos have sagged shouldn’t be a surprise, and it points toward the fundamental challenge both NBC and CBS face: Their dueling sixtysomething hosts haven’t replenished their cool factor among those under 40. And with so much competition (including time-shifted primetime DVR viewing, along with Comedy Central, Adult Swim, E! and TBS), they’re unlikely to.

NBC had to know what it was potentially sacrificing in betting on Leno, a workhorse who, at 60, will almost surely indefatigably grind onward past Johnny Carson’s retirement age of 66. Letterman, 63, previously extended his contract well into 2012, and CBS’ hopes of him staying much beyond that might hinge on two little words: President Palin.

Either way, welcome to latenight TV, AARP division.

Both networks thus possess a profile different from the traditional young-male skew that made latenight so profitable — one closer to Carson in his later years, when he lost younger viewers to Arsenio Hall.

As a side note, those demographics provide one more reason for broadcasters to press advertisers to acknowledge adults over 49. Otherwise, for all their ratings spin, the networks are really just fighting for scraps — jousting over whose fraction of a rating point is bigger.

Barring a health problem or unforeseen scandal (and Letterman has already weathered one of each), the status quo should prevail for a while. Moreover, given the fast-evolving TV landscape, tough choices analogous to the “Love me or lose me” threat NBC faced of a rival poaching O’Brien in 2004 could become all but moot before such a scenario arises involving later-night hosts Jimmy Fallon and Craig Ferguson.

That leaves everyone coexisting rather peacefully, barring the obligatory booking skirmishes over who lands guests first. Conan will carve out his new cable terrain, while Leno and Letterman keep going toe-to-toe like two gnarled old prizefighters, with Dave occasionally jabbing a symbolic thumb into Jay’s eye, well, just because.

All told, these ingredients don’t quite add up to a “latenight battle” anymore. More like a multisided shoving match.

And that’s probably just as well. Because while well-wishers may tell O’Brien to break a leg, looking at latenight’s demo drift, the real danger is that if the jockeying gets too vigorous, somebody may fall and bust a hip.