Chell's second playthrough

Chell wakes up. The same grey room. The same puzzles. Had it just been a dream? Her freedom, the wheat field, the blue sky.... The moon? Was that even real? Something isn't right. Everything that is happening to her, it had happened before. Was it time travel? We're her memories of the happening events just her imagination? Whatever it was, she somehow knew, she just knew, that she had to prevent the core transfer.
But how do you change the story that's already been made for you?