Total newbie question. Windows laptops usually have a sticker with the software license key right on the laptop. No matter how many times the laptop changes hands or how many times the OS gets upgraded, one can always return to the OS that was pre-installed (provided one can get access to the install media) by using the license key from the sticker. So how does OSX work? If I buy a used Mac laptop and it has a newer OS installed than what was originally on the machine when sold, could that original OS be re-installed? How would one go about doing that?