The whole point is that nothing will change, as far as the consumer experience is concerned. You won't get bothered about upgrades or anything like this. All that would happen is you might see an "Upgrade" button in your list of apps you purchased. That's it.

Note that while the notion of paying once and getting infinite updates forever is quite nice, it's not rooted in reality. Either the app you're using will get abandoned or you will have to pay up at some point in the future, somehow - ads, IAP, etc.