From what I've read, the second series will deal with the back story. (No doubt this could change though.) So yes, it's likely the film cast will feature in the series too. I don't see that as a problem though. I know that people who make the transition from series to film often don't come back (although there are notable exceptions. Kiefer Sutherland for example.) but the very fact this project will be both should be quite a novelty. Sure it'll be a big project, but no doubt the big stars will get paid accordingly.
Personally, I'd rather they go with unknowns but I can understand the logic why they might not. Personally, I've always been more drawn to genre and story rather than actors but I know many do not. (Often coincidentally the same actors turn up in various films though!)