BACK TO THE FUTURE (1985)
Eric Stoltz played the role of “Marty McFly” for four weeks before Steven Spielberg decided that he was too serious for a comedy. He was replaced by Michael J. Fox.
BLOOD FROM THE MUMMY'S TOMB (1971)
Peter Cushing dropped out after only one day of filming (due to the death of his wife). Andrew Keir replaced him. Director Seth Holt died and was replaced by Michael Carreras for the final weeks of production.
CURSE OF THE CAT PEOPLE (1944)
Director Gunther Von Fritsch was replaced by Robert Wise.
LORD OF THE RINGS: The Fellowship of the Ring (2001)
Stuart Townsend played “Aragorn” for four days before Peter Jackson decided to replace him with an older actor (Viggo Mortensen).
MARK OF THE VAMIRE (1935)
Cinematographer James Wong Howe was one of the best DP's in Hollywood; but Elizabeth Allan hated him. She felt that Howe was paying more attention to Carol Borland's character than her own, so she had him fired. William Daniels completed the film.
THE MASK OF FU MANCHU (1932)
Gertrude Michael was replaced by Karen Morley, and director Charles Vidor was replaced by Charles Brabin.
THE MYSTERIOUS ISLAND (1929)
The film began production in 1926, under the direction of Maurice Tourneur, with Conrad Nagel as the romantic lead. Tourneur was fired shortly thereafter, and was replaced by Benjamin Christensen. The film shut down after a hurricane destroyed the location set on Catalina Island, and when it resumed filming in 1929, Lucien Hubbard was now directing, and Nagel had been replaced by Lloyd Hughes. (Marceline Day might also have been in the original Tourneur cast).
THE OLD DARK HOUSE (1932)
Russell Hopton and Walter Byron were replaced by Melvyn Douglas and Raymond Massey.
THE PHANTOM OF THE OPERA (1925)
John Sainpolis played the role of Comte Philippe de Chagny. But when the film was rereleased in 1929, new scenes were shot in which he was replaced by Edward Martindale.
PLANET OF THE APES (1968)
Edward G. Robinson fell ill and was replaced in the role of “Dr Zaius” by Maurice Evans.
Depending on whom you talk to, director Tobe Hooper may or may not have been replaced by Steven Spielberg. The cast and crew insist that Hooper was the only director; but rumors still abound claiming that Spielberg replaced him because he felt Hooper's work was unsatisfactory. DGA rules strictly forbid producers replacing directors. So if Spielberg (who produced POLTERGEIST) did replace Hooper, he would be liable for a huge fine from the DGA.
RETURN OF THE APE MAN (1944)
Originally cast as the Ape Man, George Zucco fell ill shortly after filming began and was replaced by Frank Moran. A few stray shot of Zucco remain in the film.
SIREN OF ATLANTIS (1948)
Three directors walked
off the film (John Brahm, Arthur Ripley and William Dieterle) before
Gregg Tallas completed directing. Apparently, nobody could work with Maria Montez.
I VAMPIRI (1956) and CALTIKI THE IMMORTAL MONSTER (1958)
Director Ricardo Freda was replaced on both films by cinematographer Mario Bava.
WICKED STEPMOTHER (1988)
Bette Davis shot 11 minutes worth of footage before walking off the set in a huff. Rather than reshoot her scenes with another actress (both Lucille Ball and Bea Arthur were considered), director Larry Cohen rewrote the script and made it appear that Davis's character (a witch) had changed herself into a younger woman. Barbara Carrera played the younger version.
THE WIZARD OF OZ (1939)
At least six directors contributed to the film. Richard Thorpe began the production, but was quickly replaced by George Cukor. Norman Taurog and Mervyn Leroy shot scenes before Victor Fleming was hired, and he filmed most of the movie (and receives sole screen credit). King Vidor completed the film, after Fleming left to shoot GONE WITH THE WIND.
Are there any other examples from genre movies?