Suppose that I develop an app for the iOS platform. The app is a huge success and 80% of my target audience buys it. My customers understandably expect continuous development of the app, but my situation is difficult, because with the market saturated, I can hardly acquire new customers.

Is there a possibility to offer my existing customers a paid update of my app (e.g. buying Office 2010 does not entitle the customer to Office 2013, only to service packs, security updates, ect.) or do I need to release the update as a separate update (and is this allowed in Apple's ToS)?