It's not that hard to "upgrade" an iPhone 5c, at least in a certain fashion.
Mobile Geeks has the story of how the tech markets in the North Huaqiang district of China offer a very specialised and specific service for iPhone 5c users. Specifically, if you hand over an iPhone 5c, they'll turn it into an iPhone 5s. Well, sort of.
The process involves ripping out the motherboard from the iPhone 5c, which is the only part that's needed, and then essentially dropping it into the casing for the iPhone 5s, making the cheaper iPhone 5c appear to be the higher end model.
China being the home of iPhone manufacturing, along with just about every other bit of tech manufacturing on the planet, the spare parts aren't that hard to come by, although it's noted that the preference isn't to use the iPhone 5c's original screen, but instead use a third party screen at a cost of 120 Yuan, or around $24. The one thing that the "fake" iPhone 5s totally lacks, however, is integrated TouchID.
The odds are reasonable that you're not likely to be strolling around the technology markets of North Huaqiang any time soon, but it does also bring to light the way that fakes of all types are made if you're hunting around online for an iPhone bargain. Be aware that the next smartphone you buy might have quite different innards to the phone you'd be expecting to own.