Upgrades and downgrades
Let's say that you have a lot of pictures, documents and other files, and you don't have any space left on your hard drive.
Instead of buying an external hard drive that you have to remember to carry around, you can simply replace the internal hard disk with a bigger one. Many would even buy a new computer for this reason!
Another example: if your computer starts to slow down (especially after installing a new program or the latest updates), in most cases you have to increase your RAM memory as your computer needs to deal with an increased amount of work. Adding RAM is a quick and simple upgrade that increases the speed of your computer and lets you run more programs simultaneously.
When your computer does not support a new device or program, in most cases you have to downgrade, meaning that you must roll back to an older version of the operating system or program that you need to use.
You can also choose to downgrade when you are not familiar with the new version of a program / operating system.
For example, many people have chosen to go back to Mircrosoft Windows 7 after buying a new computer with Microsoft Windows 8.