They won't be rewriting it. She's been in Sydney for weeks. This reveal - if it's true - was bound to happen sooner or later.
I love Noomi as an actor but felt she was mis-cast in the role of Shaw. I don't feel that she brought any of her acting strengths we've seen her perform elsewhere, particularly 'The Girl with the Dragon Tattoo'; especially given the material she had to work with in 'Prometheus'.
Nonetheless, I'm glad we will see her return in her role to lend some much needed continuity between 'Prometheus' and 'ALIEN: Covenant'. So glad to hear this News overall. I don't think it will surprise anyone that she's appearing (reprising) as a major role for the new movie.
...now just waiting for this News confirmation from Official Sources, that is, 20th Century Fox.