NBC will reveal that Natalie Morales, a 9 a.m. anchor of “Today,” will replace Ms. Curry as the news anchor, and Savannah Guthrie, a White House correspondent and MSNBC anchor, will become the 9 a.m. anchor.

Ms. Vieira’s decision to leave was entirely her own, by all accounts.

Ms. Curry, who has been the news anchor on “Today” for 14 years, is well known and well liked by viewers. NBC executives and outsiders with experience in morning television see the shift as both logical for the show and to be expected by its big audience. But they agree that any change in the familiar cast of “Today” carries risk.