Katie Couric visiting "Good Morning America." (ABC/ABC VIA GETTY IMAGES)

Katie Couric, former sweetheart of NBC News’s “Today” show, will co-anchor ABC News’s “Good Morning America” next week, and probably nuke “Today’s” more than 16-year run as the most watched morning infotainment-show.

“Who can we get to guest-host?” lifestyle anchor Lara Spencer faux-wondered on “GMA” Thursday morning, as she and co-anchor Robin Roberts pretended to ruminate on who might fill in next week while Roberts is on vacation.

“We gotta find someone who has news chops, someone who can handle a tough show like this,” Spencer said.

“Maybe someone’s who’s done this before?” Roberts suggested.

“Maybe someone, who, I don’t know - she should be perky. . .” Spencer insisted.

“Any guesses?” Roberts asked viewers.

Without waiting to hear our answer, Roberts roared, “KATIE!” as giant photos of Couric appeared on screen.

Almost simultaneously, Couric began to prattle happily on Twitter about her upcoming role in ending the longest winning streak in TV history:

“Rise + shine! Excited to be back on morning TV next wk, guest hosting @GMA w/ @GStephanopoulos. #KatieOnGMA.”

“Serious pressure filling in for the fab @RobinRoberts! #KatieonGMA”

“Thanks for all the kind words about my guest gig on @GMA! I’ll be keeping Robin’s seat warm next week - Can’t wait, George! #KatieOnGMA”

Staffers at “Today” were shocked and awed by the announcement of Katie’s guest gig in a non-sweeps week, Fox News reported Thursday.

That in itself would be shocking, given that:

1.) It’s about time for ABC parent Disney - which signed Katie to star in a syndicated daytime talk show last June - to start warming up the daytime crowd for the show’s fall launch.

2.) It’s no secret that Roberts was scheduled to go on vacation next week. (Anchor George Stephanopoulos took his vacation last week.)

3.) In January, ABC News President Ben Sherwood told a ballroom full of The Reporters Who Cover Television that his goal is to “topple” the “Today” show, ending the vice-like grip it has had in the morning ratings race for “10 billion weeks.”

Sherwood was not over-promising: “GMA” has been gaining viewers while “Today’s” ratings have slid. Viewers aren’t taking to

“Today’s” co-anchor (and former news reader) Ann Curry the way they did to the show’s former co-anchor, Meredith Vieira, who left last summer, and to Couric before her. Meanwhile, NBC News is taking forever to close a deal with much-liked “Todeay” anchor Matt Lauer to remain with the show. Some sources, and news reports, say Curry’s presence is a sticking point.

“GMA” just enjoyed its closest first-quarter numbers relative to “Today” in 17 years. Last week, “GMA’ came in only about 140,000 viewers shy of “Today.”

Last time “GMA” beat “Today” for a week was in December of 1995. Once, seven years ago, “GMA” got real close, but “Today” outmaneuvered the ABC show by pulling some ratings hanky-panky to eek out the win, the New York Times noted Thursday.

What “Today” did was to put all its national ads in the higher rated first hour, and none in the lower rated second hour. Nielsen only rates timeslots that have national ads, so “Today’s second-hour ratings for that week in 1995 are lost to history. (NBC does similar things to massage ratings for its Olympics coverage. And, in fairness it’s not the only network that plays these kind of ratings games.)

But, if “Today” tries to out-finesse “GMA” next week, like it did seven years ago, it’ll be tangling with the master — longtime “Today” exec producer Jeff Zucker is now Katie’s talk-show exec producer.

Thursday may have given “Today” a glimpse as to what it’ll be like next week. On “Today,” Lauer landed an interview with America’s favorite TV bad boy, Charlie Sheen, about the actor’s very public meltdown; his upcoming FX series, “Anger Management”; and how he wishes “Two and a Half Men” creator Chuck Lorre and studio had “taken better care of the child left behind.”

These days, a Sheen appearance usually gets picked up and picked apart in the press. On Thursday, they barely noticed, because all the talk was of Katie.

Ironically, in addition to battling her former home-show, Katie will also compete next week for viewers with Oprah Winfrey — the woman she’s trying to replace as queen of daytime talk TV. Winfrey’s scheduled to appear Monday on CBS’s morning infotainment show, “CBS This Morning,” co-anchored by her BFF Gayle King.