Selfhosted

Not at all, Windows 10 and beyond identify your hardware composition, so as long as you don't swap the mainboard, the license is automatically applied again upon internet connection (assuming you install the same windows version and do not enter another license key during installation). If you want to transfer your data though, get a cheap external USB HDD enclosure and use clonezilla to clone the drive. Edit: typos

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

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.

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

The license is tied to the motherboard and hardware, so you don't need to do anything to transfer it. Just make sure you install the same version (ie; 10 Pro) if doing a fresh install.

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.

Just in case it doesn't carry over, run this first to get your Windows Pro key: https://www.nirsoft.net/utils/product_cd_key_viewer.html

As far as I know, Windows 10 licenses are on the motherboard/BIOS, so changing the harddrive should just work.

I have heard some talk about the license getting disabled if you switch some hardware in machines as it no longer can identify it as the same machine.

My (non-digital/non-microsoft account) windows pro license deactivated when I did ssd swap about a week ago (but I did a full bit copy then expanded the file system).

Let's just say I didn't buy a new license after that.

massgrave.dev

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