Shailene potentially being recast doesn't really concern me. This kind of things are pretty typical within Hollywood. Sometimes as extreme as leading man roles.
Eric Stoltz originally played Marty Mcfly in Back to the Future and a good 3rd of the film was shot with him in the role, I think all the way to when he arrives in 1955 for the first time. The only reason he was cast, I think, was because Michael J. Fox was busy on his TV show and couldn't take the role because of contractual agreements. Once he was ok'd they dropped Stoltz and that's how movie history was made.
I don't think anyone would disagree that recasting Marty was a bad decision. And whereas I think Shailene looks very age appropriate for this movie's version of MJ, I still can't help but think she's not quite what I had in mind. I have no doubt that she'd do great, but for some reason I've always been doubtful.