New
#11
First, assuming the new computer has Windows 10 on it (and it should), all you have to do is upgrade it to the latest version of Windows 10.
Second, from there, use the key to your Retail Pro version of Windows 10. Hopefully, you've kept it in a safe place and haven't lost it.
Most likely you'll have to call Microsoft to straighten this mess out because, as you know, your license is for one computer and since you can't "deactivate" Windows 10 on the old one, well . . .
Once Microsoft pegs you as "illegal", you should see a phone number for calling Microsoft. Tell them exactly what happened with as few words as possible, and things should go good from there.
Finally, if you buy a computer with Windows 10 Pro on it, you're good to go and shouldn't have to jump through any more hoops.
And a PS: Since the old computer is a desktop, and if you can find the key to your retail version of Windows 10, you can buy a new hard drive and install Windows 10 Pro on it.
Last thing: Get Macrium Reflect Free and back up your computer regularly! Then, if something like this happens again, all you have to do is Restore Windows 10.
BTW, I'm not a fan of anything that prevents my getting into my hard drive without having to jump through hoops.