Diane Sawyer will be replaced by David Muir at ABC's 'World News'

Diane Sawyer's replacement David Muir will start working as top anchor on Sept. 2. Diane Sawyer will still do prime-time specials and interviews at ABC.

ABC News is making a generational change at the top of its evening newscast, replacing Diane Sawyer with 40-year-old understudy David Muir in an attempt to take a run at longtime ratings leader Brian Williams at NBC's "Nightly News."

ABC also announced that George Stephanopoulos will add the role of chief anchor for live news events and election nights to his current jobs as "Good Morning America" co-host and host of the Sunday morning "This Week" political show.

The exit of Sawyer, 68, is not unexpected. She will remain at ABC News to concentrate on doing prime-time specials and landing newsmaking interviews, where theoretically she'll have less internal competition with Barbara Walters semi-retired and Katie Couric now at Yahoo.

When Muir starts as "World News" top anchor on Sept. 2, it makes that role a men's club at the broadcast networks again, as he joins Williams and Scott Pelley of the "CBS Evening News."

In a plugged-in world, network evening newscasts are a vestige of another time. Yet the three broadcasts together averaged 21 million viewers last week, with the audience usually smaller in the summer. ABC has been a steady No. 2 behind NBC for years, even predating Sawyer taking over as anchor in 2009, but has made inroads in recent months among younger viewers.

Two years ago ABC's "Good Morning America" toppled another longtime champ at NBC and new ABC News President James Goldston has his sights set on doing the same in the evening.

Muir has been groomed for the job. He steps in as anchor when Sawyer is away, as he did last week, and once anchored the weekend newscast. His "Made in America" reports give him a high profile on "World News" and travel around the country.

From Syracuse, New York, Muir swept floors as a teenager in a local television newsroom and got his first on-air job in that city. He is a graduate of Ithaca College, just like Bob Iger, head of ABC's parent Walt Disney Co.

"On this incredibly humbling day, I think of the 12-year-old boy with a dream of being a reporter and seeing the world," Muir said in a statement. "I never could have imagined seeing that world with Diane Sawyer."

ABC didn't make news division President James Goldston or any of the anchors available for interviews.

When Muir is host, "World News" tends to be faster-paced with more stories, said Andrew Tyndall, a news consultant who monitors the content of evening newscasts.

"It's the same newscast with a different face," Tyndall said. "I don't know what Muir brings to the newscast that Sawyer wasn't bringing."

Tyndall infuriated many at ABC last winter with a report that criticized the "Disneyfication" of "World News." He said the broadcast has become more focused on consumer and entertainment news and less on global concerns, preoccupied with what people are talking about rather than what is happening.

With his role at the helm during big news events, Stephanopoulos will take over a duty traditionally held by the evening news anchor. ABC does not want to disrupt the team of Stephanopoulos and Robin Roberts that has led the network to the top in the lucrative morning lineup. The new duty guarantees him the chance to be involved in the more substantive news stories that interest him.

He'll continue at "This Week" two or three times a month, with Martha Raddatz filling in during weeks Stephanopoulos is off.