I feel if they do turn this into a franchise, unless they plan on hiring a bunch of different actors to portray the characters as they get older, then it's a safe bet what you see on screen will not be true to the source material. Ender's Game will likely stay somewhat true (disregarding the age of the actors obviously) just to play it safe and get their foot in the door, but if it does well I'm not going to be surprised at all if they keep the young cast and branch off from the books, or somehow make everything that happens post Ender's Game sooner than in the books. (Ender accomplishing everything before he hits 25, for example.)
I tend to separate myself from "purists." If it's good I'll be happy, whether it's true to the books or not. If I'm entertained then I'm happy, and that's about as complicated as a franchise has to get for me. I just don't want the value of any future films to suffer. I want this first film to be good on it's own terms, not a shadow of the source material. And if they were to make an alternate story line that's still good, then so be it. I still have the books to keep me company if I want to revisit Card's version of the story, and I'll have the films as an alternative.
I know some may not be able to fathom what I just said, but oh well. I don't really care! :p