Something similar happened to Whoopi Goldberg when she was hired for Star Trek Generations. They said she would be actively working with Nichelle Nichols, who ultimately had nothing to do with the project. In the opening scene when she is rescued from The Nexus by Chekov her reaction of shock is genuine because she was told up until that point that it would be Nichelle who was starring in that scene and was genuinely surprised to be across from Walter Koening instead.
Hollywood sucks.