Now that the iPhone 5 is discontinued I would assume that eventually they will completely run out of them. So my question is how could we track to find out what and when Apple will start replacing the iPhone 5 w/ AppleCare with a different device, ie. iPhone 5S?
I don't really think this will happen. Apple always seems to have a magical supply of older phones on hand for warranty repairs and replacements. Once your Apple Care coverage is up, they'll just encourage you to buy the latest and great phone.