Gedeon Maheux:

I’m sure many users are expecting developers of popular applications to simply update interface elements, compile some code and easily drop a brand spanking new version of their app onto the App Store for free. There’s little doubt that the majority of iOS 7 updates to existing apps will be free (which will please Apple), but I suspect there will be a surprising number of developers who will use the launch of the new operating system to completely re-boot their app, and why not? The visual and interactive paradigms iOS 7 mark a natural breaking off point and a perfect opportunity to re-coup costs. Some existing paid apps might even adopt an iOS 7 only strategy which means they’ll have no choice but to charge again.

This makes sense. One more reason why Apple will need to clearly and strongly highlight iOS 7 apps on the App Store.