What's your plan when you release the next major version of OmniGraffle? You will create a whole new App Store listing for it? Or you'll somehow have a single listing for OmniGraffle on the App Store, and people who have purchased v7 but not v8 will be able to unlock v7 but not v8 within the app?
The latter sounds like an unholy mess, the former is very annoying as a developer, since you lose all of your reviews and history, and customers generally don't really understand how to download apps they've purchased which are no longer listed on the App Store, and presumably you'd pull OmniGraffle v7 from the App Store.
Also, right now in the U.S. Mac App Store, you have 3 reviews on the current version of OmniGraffle, and 2 out of 3 are 1 star reviews complaining about the way your free trial is presented.
The Mac App Store is a broken mess.