Good news, Windows 10 fans: if you follow these instructions, your genuine Windows 7- or 8.1-based PC will always be associated with Windows 10, and you will be able to clean install the OS at will going forward, and without ever having to manually activate. It will just work.
And it’s simple enough: just upgrade the PC to Windows 10 normally. Once.
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You can upgrade using the reservation system and Windows Update. Or by using the Setup media/ISO you downloaded from Microsoft. Either way.
When you do so, Windows 10 will activate—yes, you need to be online for that, obviously—and the PC’s digital entitlement will be registered with Windows Store. What gets sent to Windows Store is interesting: it’s a hardware ID, unique to your PC, which is comprised of information about its CPU, motherboard and, if available, TPM. What’s not send to Windows Store is your Microsoft account information. Instead, it is the PC that is registered, essentially, which makes sense since you may later sell it or give it away. That PC will always activate new installs of Windows 10 going forward. (And yes, your previous OS will still activate too. This process doesn’t impact that at all.)
So what if you want to do a clean install?
Once you’ve upgraded from the previous, genuine OS, you could of course simply Reset the device as I outlined in Windows 10 Tip: Successfully Clean Install Windows 10. That will always work too. The Reset process results in a configuration that is essentially identical to a clean install.
That said, if you prefer to boot your PC using the install media you created as described in Microsoft Delivers Windows 10 on ISO, you can do that too. Just don’t enter a product key—Microsoft won’t give you one and you won’t need it either—and Windows 10 will simply activate automatically when the install is done. Because, again, that PC is registered with Windows Store. It’s all good.
What if something changes with your hardware?
If it’s something major like a motherboard replacement, its hardware ID will no longer be the same. If this was done by your PC maker or an authorized repair center, no worries: they can inject an OEM key and make sure the system remains activated and will be good going forward (because the new ID is now registered with the store). If you do this yourself, you will need to phone activate.
In other words, everything works exactly as expected. So we can actually relax now. It looks like Microsoft has done the right thing, and I’ve been told they will be documenting this information on the Windows web site soon.