What are some films that were drastically altered due to actor availability issues?

In the world of filmmaking, the availability of actors can significantly impact the final product. Sometimes, schedule conflicts or unforeseen circumstances force directors to make substantial changes to a movie's script or production. This can lead to memorable cast substitutions, rewritten roles, or even plot adjustments. One famous example is the ‘Back to the Future’ series, where the character of Marty McFly was originally played by Eric Stoltz before Michael J. Fox took over. Exploring movies affected by such changes offers an insightful look into the adaptability and creativity required in the film industry. What are some other films that experienced notable transformations due to actor availability, and how did these changes affect the stories being told?