At least on retail keys if you've updated Win7/8 machine to Win10 once, you can do fresh install of Win10 completely without a key and it'll activate once installation is done, I don't see why it would be any different on OEM keys
The problem is, when you upgrade from 7 to 10.. Windows lost the key. Replaced with special windows 10 key that is tied to the motherboard.
They call this 'hardware upgrade license' or something. It's not oem, it's not retail. It's special license that have no eula given (at least from my Google search).
So you can't reactivate if motherboard changed. Despite windows itself says it can reactivate and keeps trying to reactivate and keeps failing (this whole fiasco was told more detailed in my old thread somewhere in b3d, complete with official Microsoft response)
Edit: but if your machine pre installed with with Windows 8, your key will still be safe inside UEFI.
It can be manually retrieved using a script I posted a while back on b3d.