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.