The problem is Apple doesn't currently care about paid upgrades. They don't sell software that way (witness the new Logic).

Developers need to do things to ensure the loyalty of old customers while pricing their software so they can make a living. So the starting price + upgrade pricing model works great. Customer feel like they got a bargain for being loyal and the developer gets to eat.

Apple doesn't need to do that. So they won't build it for developers. Given the iCloud API thing, I would imagine this is going to continue to get more restrictive.