This happened to me last year and Apple were very good about it. Several keys became loose on my MBA and I took it into the store, where they attempted to fit new keys but it transpired that I had lost the clips that hold the keys in place (there's 3 parts to it), so they replaced the whole top case under warranty.
They cannot replace just the keyboard as it is moulded in, but they will attempt to replace the keys and then it's a new case.