In most cases, the developer just has to recompile the app and include iOS 7. Of course it get's a lot more tricky if one wants to start using new APIs introduced in iOS 7, and that's why most devs will drop support for older versions.
Support for iOS 7 can complicate things, but for -- I'd estimate -- 75%+ of developers it's simply tweaking a few things and then releasing an app that continues to support v5.0 and up at the very least.
This isn't as dramatic an announcement as some are making it out to be. Though maybe it should be, as the way to encourage both consumers and developers to stay up-to-date is to force them into it with limited support.
There are a bunch of new features that I’m adding. I especially like the ability to speak text with a Siri-like voice.
I think my interfaces fit into the iOS7 ecosystem, but until they start enforcing the policy on Feb 1, I won’t really know. Figuring out what the rules really mean is always a big unknown in the app review process.
The TLDR is native iOS apps are going to die pretty soon.
Native iOS apps will die when iOS dies, which appears to be about as "soon" in tech industry time as the sun running out of helium.
There are a couple implications. First of all schools (like where my kids go) that bit the apple and deployed ipads to all the kids can't install or upgrade software after February. So if you're selling apps to kids, get your profits while you can. After february if they spend all their budget on upgrading ipads to the latest hardware so they could eventually install apps, they're not going to have any money to spend on apps, so that is a double whammy on sales. Something tells me no one told the accountants and administrators that the depreciation period was so short; much like elementary schools reuse textbooks for 20 years I think they were a wee bit optimistic about ipad usable lifespan.
Another implication is if you have a use case for app-less (web based) ipads, the market is probably going to absolutely flood in a couple months with cheap ipads that can't run modern native apps. So if you're trying to develop an ipad app for cheap people, the wise choice at this time would be web based not native. Also a web based solution could be sold to android users or desktop users or whatever.
The TLDR is native iOS apps are going to die pretty soon. Which is too bad, it was an interesting technological idea.