Here's a ingenious little hack to use your iPhone 5s after you've upgraded to iPhone 6: you can install Android L Developer Preview. It really doesn't sound possible knowing Apple's tight control over device security, but it's true - a group of university researchers have managed to find a way to do it. You can see the proof in the brief demo video below.
Combining Google's open Android operating system (plus all the fantastic benefits that go with it) with Apple's amazing hardware and most agree you’d have the perfect smartphone. Nice dream, but practically not ever likely to happen, regardless of the accomplishments of some talented developers who, over the past years, have been successful in installing Android on an iPhone.
You'll see in the video below a researcher from the University of Michigan, Mike Huang, demonstrate an iPhone 5s running the Android L Developer Preview. Certainly it's not as perfect as it should be, considering the device has a 64-bit A7 processor, but this is due to the way the whole thing works.
You probably already know that the iPhone 5s doesn't have Android L's system files locally stored due to the problems with installing Android on unsupported hardware. What it does do though is to access them through a remove server, the same way a PC accesses remote files when you use a platform such as OnLive to stream games.
So yes, it's a pretty impressive feat, but it's really only for fun. It's way too slow for regular use, and its reliance on a remote server makes it unusable without a data connection.