The story centers on a lonely writer who falls in love with his new computer operating system, that is supposed to tend to his every need. Scarlett Johansson will provide the voice for this program. The cast also includes Joaquin Phoenix, Rooney Mara, Amy Adams, Chris Pratt, and Olivia Wilde. Here's what Spike Jonze had to say about the last-minute change, in an email to Vulture.
Samantha was with us on set and was amazing. It was only in post production, when we started editing, that we realized that what the character/movie needed was different from what Samantha and I had created together. So we recast and since then Scarlett has taken over that role.
I love Samantha. I've been friends with her forever and I hope we make lots of things together in the future.
It isn't known if this will affect the November 20 release date or not, but since Scarlett Johansson is only providing her voice, it seems likely the fall release will happen as planned.
Her was released December 18th, 2013 and stars Joaquin Phoenix, Lynn A. Freedman, Lisa Renee Pitts, Gabe Gomez, Chris Pratt, Artt Butler, May Lindstrom, Rooney Mara. The film is directed by Spike Jonze.