The morning-show host will be the new anchor of CBS Evening News

Saying that “sometimes change is good,” Katie Couric – after 15 years cohosting NBC’s Today show – will jump ship to anchor the CBS Evening News, she announced on Today Wednesday morning.

She will become the first woman to serve as sole anchor of a network flagship newscast.

On Today, cohost Matt Lauer had turned to Couric and asked, “Anything new?” There was.

“After listening to my heart and my gut – which have served me very well in the past few years – I’ve decided that I will be leaving Today at the end of May,” Couric replied. She called this “a difficult decision,” telling viewers, “It may sound corny, but I really feel that we have become friends over the years.”

She called her going to CBS Evening News (and 60 Minutes) “the worst kept secret in America.” Today news anchor Ann Curry said she felt like she had a “sister who was going off to college,” while weatherman Al Roker said CBS was “the lucky one.”

“Once in a while we get on each other’s nerves,” Couric said during her announcement, but, turning to Lauer, added, “As Dorothy said to the Scarecrow, I think I’m going to miss him most of all.”

Later on Wednesday, CBS confirmed that Couric will be anchor and managing editor of the evening news, as well as do prime-time specials and contribute to 60 Minutes as part of the five-year deal.

It’s been her lifelong dream to do 60 Minutes, Extra executive producer Lisa Gregorisch-Dempsey, a close friend, tells PEOPLE. “She used to watch it on the bedroom floor of her parents’ room, eating leftover fried chicken.”

The news of Couric’s departure from Today – which comes 15 years to the day after she debuted on the top-rated morning show – ends months of speculation about whether Couric, 49, whose four-year, $65 million contract at NBC expires in May, would decide to fill the evening anchor chair left open after Dan Rather retired last March.

News veteran Bob Schieffer has served as the temporary anchor of CBS Evening News, currently ranked third among the three major network newscasts.

On Tuesday, Schieffer said, according to The Washington Post, “I’ve known Katie for many years. She’s a first-rate reporter and a terrific interviewer.” He added, “I love Katie, and I think she’ll love CBS News.”

In January when rumors began surfacing that Couric was being considered for the slot, she addressed the speculation by telling reporters: “My contract (with NBC) ends in May, and I’m trying to figure out what I’m going to do I’m fortunate to have a couple of opportunities to think long and hard about.”

The likeliest replacement for Couric on Today is said to be Meredith Vieira, currently one of the hosts on The View and the host of the syndicated Who Wants to Be a Millionaire, both part of ABC.

Others mentioned have been NBC’s Campbell Brown, Natalie Morales and Ann Curry.

Reacting to all the buzz about Couric that was circulating Tuesday, ABC’s Diane Sawyer told TV’s Access Hollywood she was “truly, truly excited,” adding, “I’m just so happy when women are in strong positions everywhere.”