Well, it won't be a "demo". What's getting released will be roughly a fourth of the game, but it will be pretty much final. The plot is setup up in such a way that it sort of takes place over four "arcs". It's a political story, so Arc 1 is how the characters get involved, Arc 2 is about the politics, Arc 3 is about the conflict/war that breaks out, and Arc 4 is the conclusion.
So when I release the first fourth, it'll be done, ready for release, final and not really subject to revision save for bug fixes and suches.