Doug Gresham was asked about it recently and he said that people needed to worry more about global warming than the issue of Susan and Caspian. I don't think it will be a big thing. They've got to know by now that the fans would absolutely hate that.
Remember though, that Susan didn't know she wasn't going to return till the end and that she may have thought that she could stay like she had before...who knows. I don't think it will be a problem. We'll have to wait and see what happens once it comes out.
Good to hear a new person's view Jess, again welcome to the forum!