It would be a lot of footage to replace for what would basically be (in my opinion, it’s subjective) novelty value; I’d love to see a few select shots as Young Alec Guinness, though. I don’t know where the idea that he hated being in Star Wars originally came from; by all accounts he had a lot of fun on set but didn’t take it deeply seriously.
De-ageing Natalie Portman is a really cool idea. It might look a bit weird later in the movie though, when she’s not covered by robes and her limbs clearly have an adult’s proportions.