With the unibody non-retina MacBook Pros, they have a manual for doing it yourself. They also don't have proprietary 5-point screws securing the bottom case on those machines and instead just use garden-variety #00 Philips. With those machines, you can absolutely replace drives and RAM on your own with no trouble or flack from Apple. With the retina MacBook Pros, they make as many claims up front that both storage and RAM are fixed at the time of purchase (even though the SSD is removable and upgradable). You are not to open that machine up. You certainly can and not void your warranty, but they can refuse service upon seeing an aftermarket drive on grounds that it is causing trouble.
"Normal MBPs" are not "Retina MBPs". There is a distinct difference in both policy and procedure when it comes to non-retina MacBook Pros and retina MacBook Pros.
Apple can and frequently does deny service on those grounds regardless of whether or not the aftermarket part is the culprit. It's annoying, stupid, and wrong of them, but there you go.