You don’t upgrade your computer often. And once you do, you normally don’t feel the need to keep the old components, and you’ll think about disposing them – errr.. via Ebay for few $$.
Before you dispose your old components make sure you don’t need it in the future. There may be some products, which may be of no use now, but may be they’ll come handy if the new component starts malfunctioning.
Let’s suppose you install a new component and its malfunctioning, you have no idea if it is a technical glitch or some installation problem. In such scenarios, you can replace the new component with your old component ( which you are sure it’s working ), and then check if it works fine. If it does, you can be sure that it’s a technical glitch with your new component, else it most likely will be something to do with the installation problem!