Can you upgrade your macbook pro gpu from the radeon 455 to a radeon 460?

I have a 15-inch macbook pro with 2.7 GHz intel core i7 and a radeon pro 455. im sure ill be stuck with this labtop for a bit but i want to experience a better quality of gaming. turns out the model can come with a radeon pro 455 or 460. i have a radeon pro 455 but can you upgrade your macbook pro by simply going to the apple store and buy the better graphics? and if you know, how much would it probably cost?