It's perfectly acceptable to change your business model, but definitely be aware of backlash.
Recently Chargify, which makes a Web-based system that small companies use to manage billing, made some major changes to their model and it pissed a lot of people off. To me, the problem was in communication.
I don't mean communication when they made the change, but communication early on, before the change.
People have to understand that you're in this to make a living, or at least be sustainable. Communicate that to them before you implement the structure change. Make sure they always know that eventually you will move to a freemium model, especially if you will restrict the free users once the premium version is available.
Chargify should have been more open from the get-go, and communicated that as they started realizing that their business model wasn't having high enough conversions.
Don't follow their lead. Remember, you can always grandfather-in old customers.
If you're simply planning on adding content/features for premium users and otherwise keeping the app the same for standard users, you have no need to worry at all. People who love your product will upgrade for the features if they are worth paying for, and the others will simply be content with their bare-bones version.