That is a good point, is creating a new app feasible within the app store? It seems if there are enough new features added, then it would become a new app which would be chargeable as a new purchase (with a discount code for users of the existing app). Presumably, such a milestone release would only happen every 2 - 3 year so there is time to think of an appropriate addition to the title ("Pro," "Premium", "Plus", etc) or very occasionally launch a new product.