Are the enhancements you're planning with the next version separable from the rest of the site? It sounds like you answered your own question: you feel that th

Are the enhancements you're planning with the next version separable from the rest of the site? It sounds like you answered your own question: you feel that the stuff you're adding to the site will make it worthy of paying for, as opposed to your current site which presumably is less worth paying for (but it sounds like you have a significant number of users happy with your free site). Why not just charge for the new features that you're adding?