UPDATED: Muir will take over Sept. 2, while George Stephanopoulos becomes chief anchor at ABC News. After five years in the World News anchor chair, Diane Sawyer will step down at the end of the summer, clearing the way for David Muir, who anchors the program's weekend edition, to assume her role on Sept. 2. Sawyer, 68, an ABC News star who previously anchored Good Morning America and newsmagazine Primetime and who has conducted some of the news division's most-watched interviews, will head her own unit and continue to produce and anchor specials. Additionally, George Stephanopoulos, once thought to be...