App Store upgrade pricing is already here
One of the more common complaints with Apple's App Stores I read about is that Apple doesn't allow upgrade pricing. On a Mac or PC, if you've already bought version 1 of application Foo, you can often buy version 2 at a reduced price. People have been asking Apple to add support for this since forever, since the current "buy once and get updates for free forever" isn't sustainable. Developers need to be able to get more money from users to fund the additional work they're doing after the initial sale.
This all makes sense, and it would be good if the App Store supported it. But here's the thing: it already does ! Kinda…
There's something called "app bundles", where a developer can sell multiple apps at the same time for a reduced price. For example, Toca Boca is a company that makes (awesome) kids' apps. They've got one bundle called "Toca Box for Autism Awareness" that contains "Toca Store", "Toca Hair Salon", "Toca Life: Town" and "Toca Band". Bought individually the apps would cost $11, but the bundle costs $6.
Now, if you've already bought one of the apps in the bundle, there's a feature called Complete My Bundle that will deduct what you've already paid for apps in the bundle from the bundle price. Let's say you got Toca Band for $3 a while ago, now you'd only pay $3 to get the bundle and the remaining three apps.
And this is all you need to support paid upgrades. How? Just make each major version of the app a separate app, so you've got both App v1 and App v2 in the App Store. Then you make a bundle and price it at "price of v1 plus what you'd like the upgrade to cost". If your app costs $40 and you want a $20 upgrade price, set the bundle price to $60. Now if someone already has v1 they can buy the bundle and pay only $20 thanks to Complete My Bundle. And if they don't have v1 they just buy the separate App v2.
Are there any downsides? Yes, it's a bit confusing for the customer perhaps, so you might need to help them get the right bundle to upgrade. And you can only have 10 active bundles at any time, but unless you've got a lot of apps I don't think that would be a real problem.
There's some more info on bundles over at Apple's place, should you like to give it a try. Seems easy enough.