I doubt I would change the whole cast of a film, but there might be one or two actors I might switch for roles. For example, in the latest Star Trek films, I would switch Simon Pegg's role as Scotty for Paul McGillion, and swap Benedict Cumberbatch's Khan for someone else. I think Pegg is a good comedic actor, but he does not remind be of Scotty at all, either in appearance or performance. I like Cumberbatch, but again, he looks and sounds nothing like the original Khan.