The waiting begins at ABC News

Reporting from New York - Thursday night, retiring ABC anchor Charles Gibson was feted by colleagues in a Lincoln Center reception hall overlooking the Hudson River. Among the hundreds on hand, one person was conspicuously absent: Diane Sawyer, who succeeds him tonight on the network's flagship evening newscast, "World News."

She was already on assignment, headed to Copenhagen to meet with Iranian President Mahmoud Ahmadinejad for an exclusive interview to kick off her tenure. It was a move that speaks volumes about her ambitions for "World News" -- a post she has long sought.

Sawyer has declined to speak publicly about her plans for the broadcast, but news division employees say the 63-year-old anchor has spent the last months immersed in discussions about the show.

ABC executives stressed that any changes will be subtle, particularly in the early days.

"People should not tune in and expect a revolution," said ABC News President David Westin.

But with the Ahmadinejad interview, her second with the Iranian leader, Sawyer made clear that she plans to continue to pursue the kind of high-impact "gets" that have been her calling card.

Whether her competitive zeal will change the program's fortunes remains an open question.

Sawyer's ascension to "World News" marks the fourth anchor changeover on the newscast since Peter Jennings' lung cancer forced him to leave the program in spring 2005. Gibson, Elizabeth Vargas and Bob Woodruff filled in during an eight-month interregnum while Jennings was ill and after his death that August. Vargas and Woodruff were then named the new "World News" anchor team in December 2005. That pairing was cut short when Woodruff was seriously wounded during a reporting trip to Iraq, prompting Gibson's return as sole anchor in May 2006.

"We have had more turnover in our evening news anchors than probably at any time in the history of broadcast television," Westin said. "And that's not a good thing for the evening news."

But ABC officials are optimistic that Sawyer will be welcomed by the audience -- so much so that they've done little to trumpet her arrival, aside from a few promotional spots that cast her as tough-minded journalist in the mold of Jennings and Gibson.

"The viewers all know Diane," said "World News" executive producer Jon Banner. "She is no stranger."

Indeed, after 20 years at ABC, Sawyer is the network's most bankable star. But with its low-key handling of her move from "Good Morning America" to "World News," ABC is also working to lower expectations that Sawyer's arrival will be an immediate game-changer in the evening news race. It's a lesson learned from CBS' much-ballyhooed hiring of Katie Couric, which did not budge its newscast out of third place.

During Gibson's tenure, ABC at one point pulled past "NBC Nightly News" but lost ground this season and last. Almost 8 million people on average have tuned into "World News" since September, a drop of 5% from last season, while NBC has attracted 8.9 million viewers, up 1%, according to Nielsen. "CBS Evening News" trails with 6 million, down 4%.

"We certainly want to succeed," Banner said. "But for our purposes, we know this is a long-term effort, not something that is going to happen overnight. I'm much more interested in what happens six months from now than what happens six days from now."

Sawyer's selection as Gibson's successor precipitated a round of anchor shuffling inside the news division that is still playing out. Last week, "This Week" anchor George Stephanopoulos took Sawyer's spot on "GMA," a move that now requires executives to find a new anchor to host the Sunday political talk program.

Westin jokingly called the time frame for that decision a "deep, dark secret" but added that he hoped to settle on a new anchor soon to take the burden off Stephanopoulos, who is anchoring both shows in the interim. The news president said he's casting a wide net, considering candidates from both inside and outside the news division.

"Nightline" anchor Terry Moran and senior White House correspondent Jake Tapper, who have both substituted for Stephanopoulos, are considered possible successors. PBS' Gwen Ifill is also widely viewed as a prospect.

With three of the networks' major newscasts in flux at the same time, the stakes are high for ABC. Perhaps most vital is the success of "GMA," the news division's main revenue generator. A drop in ratings among 25- to 54-year-old viewers, the key demographic for advertising, could translate into a loss of millions of dollars.

During Stephanopoulos' first four days on the show, the morning program averaged 4.32 million viewers, according to early estimates, slightly higher than the show's season average of 4.24 million.

Westin said that simply keeping viewership steady initially after Sawyer's departure would be a victory.

"We know that there are many people that come to 'GMA' because of Diane Sawyer," he said. "If we hold the audience, I think that's a major accomplishment."