You transfer by installing Windows 7 or 8.x on the new computer, then upgrading it to Windows 10. Once your old key has been upgraded once (assuming it's a retail key) it will be recognized as a valid Windows 10 upgrade by the activation servers when you perform the upgrade on a new machine, even if it's beyond the 1 year free upgrade period. The license is tied to your old product key, even though you can't install Windows 10 clean the first time with that product key on that hardware.
The first time you install Windows 10 on any given hardware, you must perform an upgrade from the previous retail OS (For clarification this is only for the free upgrade, If you buy a Windows 10 license, you can of course install clean at any time). You are free to perform clean installs as many times as you want on that hardware after the upgrade.
I hope that this will be verified prior to the end of the free upgrade program.
I also hope that here's a backdoor way to install and activate Win 10 from the free upgrade without installing the qualifying OS first. With the Win 8 upgrade license, the official way was the same: install it as an upgrade from an installed, and activated, qualifying OS. There is a simple work-around for that that allows 8.1 to be installed and activated on a new system without any prior OS. I recognize that it there may be no backdoor for Win 10: the upgrade is free, and it does not produce a license key.
Indeed, this is very confusing. I found out that with Win8.1 and an mobo with a UEFI BIOS, the product key is baked into the mobo. That is why no key was needed when I upgraded my HTPC. How that works on other, older mobos without a UEFI BIOS I am not sure. So Win10 uses the baked in product key for doing a clean install (that is how it recognizes that there is already a valid license). Now can the same Win8.1 key be added to a NEW mobo and will Win10 upgrade? It comes down to the question of does Win10 do a lookup in a MS database of used Win8.1 license numbers? I think so. MS used to just use a hash mark algorithm to verify that the license was valid but obviously, MS now knows that the license was used otherwise all anyone would need is a single valid Win8.1 key and use it over and over.
I have a feeling that, like the old OEM license procedure, you will have to call MS (in India and talk with Susie) and say that you replaced a "defective" mobo in your original computer and you need to have the Win 8.1 license number renewed. That should allow you to do another upgrade to Win10 with new hardware. And you may be able to do it long after the one year "free upgrade".
Just to add to the confusion, I have Technet Win8.1 Pro Retail licenses that are good for 10 activations! My mind boggles.
BTW, I just ran Keyfinder on both my Win10 insider and my upgraded HTPC (using the MS MediaCeationTool.exe program. Both have the same revision number (ends in 363) and both have the generic Win10 Pro license number that came with version 240:
I still allow that a retail Win 7/8 key registered for the free upgrade may allow a direct Windows 10 upgrade. If the key is registered with Microsoft servers as valid for the free upgrade, beyond the 1 year period, then I can't see why it wouldn't be recognized during the process of a clean installation, as it is being recognized during an upgrade just fine.
I'll be waiting to see confirmation from someone on this. The key is that the license must have gone through the upgrade process at least once before, to be registered as a Windows 10 license. Then I hope it would work on new systems, for a clean Windows 10 installation.
My computer's motherboard has a UEFI BIOS, but Windows didn't come pre-installed on my computer... it was a system builder copy I bought and installed, since Microsoft had already pulled the retail copies from shelves.
So, is the Windows 7 key still baked in it? Or is that only if it comes pre-installed on an OEM machine?
Of course, I could be wrong about all of this but its sounds reasonable, heh.