The system should activate itself if you only replace the disk.

But… if you ever need a license use one of the activation scripts from massgravel on githu… I mean buy a new license on microsoft.com and enjoy your shiny, legally activated operaring system.

…or just install Linux, if your use case allows you to do so

So some fun facts:

• If you buy enough licenses from Microsoft, instead of giving you a bunch of unique licence keys to keep track of, they will give you a license that you can install on a server, and a special “volume license key” that you use on every machine - then, instead of talking to Microsoft to activate themselves, they connect to your server which ensures that it is only activating as many machines as you have licenses for
• These volume license keys are public knowledge to the point that Microsoft publish them on their site because they are useless unless you have a server to validate the activations
• The server protocol is not complex, has been reverse engineered, and there are open source server implementations that forget the whole “ensure you have the right number of licenses” part

If you install fresh using a USB created by the Media Creation Tool, there is one detail you’ll need to know. The install/OOBE setup will still prompt you to enter a key and activate. Just tell it to skip for now/activate later. Once it finishes and goes online, it will activate automatically.

How new is the optiplex? If it shipped with windows 8.0 or newer then the key is embedded in the bios and there is no transferring of the key.

If it shipped with windows 7 then just activate it like normal over the phone and it should work.

If you’re replacing, why not clone the old drive to the new and expand to fill?