The changeover at the National Geographic Society and NPR continue. NPR CEO Gary Knell today was announced as president and CEO of Nat Geo Society, leaving NPR after 21 months on the job. At NGS, Knell replaces John M. Fahey, who had been CEO since 1998, adding a chairman title in 2011. Fahey will continue to serve as chairman of the board. Knell will transition to the new position in the fall. Fahey has been shaking up the upper ranks at the 124-year-old nonprofit scientific and educational institution over the past couple of years in an effort to bring it faster into the digital age as well as modernize its TV operations and expand production capabilities. NGS president Tim Kelly left at the end of last year, while Brooke Runnette replaced Maryanne Culpepper as president of National Geographic Television. Meanwhile, Knell’s departure from NPR extends the revolving door at the top of the embattled public radio network. His replacement is believed to be the seventh permanent or acting NPR CEO in just over seven years. Knell succeeded Vivian Schiller who was forced to resign over a string of controversies. Before joining NPR, he spent 22 years at Sesame Workshop, including a 12-year tenure as CEO.