In making the move, Mr. Lack is rewarding Mr. Oppenheim, who in less than two years has brought “Today” back into a competitive position with ABC’s “Good Morning America.” NBC News’s most significant properties — “NBC Nightly News,” “Meet the Press” and “Today” — are all in good shape, and it will be Mr. Oppenheim’s job to keep them there. He’ll also have to find a way to integrate Megyn Kelly into NBC’s morning schedule, where she will face huge expectations and tough competition.

In his memo, Mr. Lack said that “one of the signs of a healthy and thriving newsroom is the ability to find a worthy successor within your own walls.”

Ms. Turness, a 49-year-old native of Britain who was recruited to NBC from ITV News in Britain in 2013, will return to Europe to help manage an initiative that is important to the network and Mr. Lack. (Phil Griffin will remain in charge of MSNBC.) Ms. Turness’s move removes the top female executive in television news from the hierarchy at NBC headquarters; when Mr. Lack arrived two years ago to take over NBC News, he succeeded another woman, Pat Fili-Krushel.

NBC’s 25 percent stake in Euronews means that the network can, at long last, tap into an international market. This will enable it to compete more aggressively with CNN, which has a robust international presence (Euronews will be renamed Euronews NBC). Founded in 1993, the network has more than 400 journalists and is broadcast in 13 languages.

Comcast and NBC Universal do not have a big international presence, and the company had been looking for an expansion plan. Though Euronews has a wide footprint — it reaches 277 million households across Europe, Africa and the Middle East — its viewership and influence are small. As part of the Euronews move, Mr. Lack told staff members that NBC would “invest in European journalism, strengthening the depth and pace of its reporting.”