It's been an honor and a privilege bringing you the news every night. And the only thing that makes this transition any easier is knowing that you will be in very capable hands.

I'll see you again tomorrow. For all of us at ABC News, goodnight"

ABC News President David Westin said Gibson's move to "World News Tonight" was driven by Vargas' pregnancy and Woodruff's recovery from severe wounds inflicted by a roadside bomb blast in Iraq on January 29.

Gibson, who joined ABC News in 1975, will continue to co-anchor "Good Morning America" with Diane Sawyer and Robin Roberts until June 30, when he will leave that show for good to devote himself exclusively to "World News Tonight."

In a news release, Woodruff called Gibson a mentor and friend, and said, "I look forward to contributing to his broadcast as soon as I am able," the Associated Press reports.

Gibson, said he was "humbled to accept this new assignment." He will begin his new job on Monday while continuing on GMA through the end of June.

Meanwhile sources tell us ABC's move was driven largely by CBS beating them in the ratings even though "Today" veteran Katie Couric is not in place as yet. Vargas is also a reported diva.

The latest shuffle at ABC News, comes months after co-anchor Bob Woodruff was wounded in Iraq.

It also comes as ABC's "Good Morning America," co-hosted by Gibson, seemed poised to catch up with rival NBC's top-ranked "Today" show in the breakfast-hour ratings race