The TCM.com article about this movie says:

Wood's death...left the production...with an uncertain future. While there were still three weeks of principal photography left, she had already completed filming all of her major scenes. According to [director] Douglas Trumbull, there were only two more scenes left for Wood to shoot, and they were minor.

Per Wikipedia:

Trumbull proceeded to complete the film by rewriting the script and using Natalie Wood's younger sister Lana for Wood's few remaining scenes.