How did RKO alter The Magnificent Ambersons after poor test screenings?

Following poor test screenings of The Magnificent Ambersons, RKO took control of the film from Orson Welles to address concerns regarding its length.12

The studio ordered Welles’s editor, Robert Wise, to significantly shorten the film, resulting in approximately 44 to 50 minutes of footage being cut. Additionally, RKO had assistant director Freddie Fleck and Wise shoot a new, happier ending to replace the original conclusion.12

Welles, who was working on a project in Brazil at the time, unsuccessfully attempted to stop these changes, which he famously characterized as the "butchering" of his work. RKO subsequently destroyed the negatives of the deleted material, and the original cut of the film has never been recovered.1