Can a retail version of Windows 10 that was upgraded for free from retail 8.1 be transferred from my current system to a new one? i.e. Is it possible/within the EULA. If so what is the best was to go about it, assuming the new system will use a different MS account (so the activation won't be transferred digitally with that)? Can I activate using the appropriate 8.1 key? I've tried googling the answer but found mixed messages.
As far as I'm aware, yes. You may need to go through the automated phonecall bollocks with it being an existing key on new PC, but unless MS have changed their minds, you should be able to install and activate it on the new PC with the 8.x key.
As it’s retail, yes. You’re fine. You would just need the key, unless you linked it to a Microsoft account when you activated it previously.