Existing apps will work just fine. However building with XCode 5 is probably going to enforce the 60x60 requirement and Apple will likely reject apps that come in at some point without it. Clearly if you build for the iOS 7 SDK, you are going to have to provide the new Icons. Hopefully it will be smart enough to look at the build target(6.1, 7.0) and decide what Icon’s it needs. I’ve not tried to do a device build recently. I need to get the latest daily so I can build against the 7.0GM and see.
We have already updated the build.settings guide to cover what we are officially recommending for a naming scheme. Apple gives developers some wiggle room and there may be a Corona requirement to actually have the Icon.png at 57x57. You can see that now at:
http://docs.coronalabs.com/guide/distribution/buildSettings/index.html
We are contemplating a blog post to cover this and so some other iOS 7ism’s.
One of the reasons we are going with the names we have, is most developers will only need to add icons, not change/rename any existing ones. It’s also very explicit and the less flexibility, the less room for error for many of our less experienced developers.