Several years before writing Changeling, television screenwriter and former journalist J. Michael Straczynski was contacted by a source at Los Angeles City Hall. The source told him that officials were planning to burn numerous archive documents, among them "something [Straczynski] should see". The source had discovered a transcript of the city council welfare hearings concerning Collins and the aftermath of her son's disappearance. Straczynski became fascinated with the case; he carried out some research, and wrote a spec script titledThe Strange Case of Christine Collins. Several studios and independent producers optioned the script, but it never found a buyer. Straczynski felt he lacked the time to devote to making the story work and only returned to the project following the cancellation of his television show Jeremiah in 2004. After 20 years as a screenwriter and producer for television, Straczynski felt he needed a break from the medium, so he spent a year researching the Collins case through archived criminal, county courthouse, city hall and city morgue records. He said he collected around 6,000 pages of documentation on Collins and the Wineville murders, before learning enough to "figure out how to tell it". He wrote the first draft of the new script in 11 days. Straczynski's agent passed the script to producer Jim Whitaker. He forwarded it to Ron Howard, who optioned it immediately.
In June 2006, Universal Studios and Howard's Imagine Entertainment bought the script for Howard to direct. The film was on a shortlist of projects for Howard after coming off the commercial success of The Da Vinci Code. In March 2007, Universal fast-tracked the production. When Howard chose Frost/Nixon and Angels & Demons as his next two directing projects, it became clear he could not direct Changeling until 2009. After Howard stepped down, it looked as if the film would not be made, despite admiration for the script in the industry. Howard and Imagine partner Brian Grazer began looking for a new director to helm the project; they pitched the film to Eastwood in February 2007, and he agreed to direct immediately after reading the script. Eastwood said his memories of growing up during the Great Depression meant that whenever a project dealing with the era landed in his hands, he "redoubled his attention" upon it. Eastwood also cited the script's focus on Collins—rather than the "Freddy Krueger" story of Northcott's crimes—as a factor in deciding to make the film.