These are being sold on Apple's AppStore, and there the model is that you get all of the updates for that App. Of course there is the work-around that some apps use, which is to create a new App (i.e.: MyApp vs MyApp2), which Apple could do at some point in the future.

The best one to watch at the moment is if Pixelmater Pro license holders from before it was bought by Apple get access to any of the new improvements.