Although there once was a time when TV newscasters held on to their jobs for generations, such is no longer the case. ABC announced Tuesday that Charles Gibson, who began anchoring ABC World News in May 2006, will leave the nightly newscast on Friday, December 18. He succeeded Elizabeth Vargas, who took her seat at the anchor's desk in January of that year (along with BOB WOODRUFF, who was soon seriously injured while on assignment in Iraq). Gibson will be succeeded on Monday, December 21 by his former co-host on Good Morning America , Diane Sawyer. No permanent replacement for Sawyer on GMA has yet been announced, but following the death of Peter Jennings in 2005, Sawyer and Gibson took turns performing double duty on both the morning talk show and evening news show.