So I currently have a Windows 8 Pro key that I bought in 2012 at Microsoft's official store for €30 (promotion for eligible people who had xp, vista, or win 7 - I had win 7), which further down the line gave me the free upgrade option to windows 10. It's only been used on the computer I am using now (so only 1), and when I do "slmgr -dli" in cmd, the prompt shows: "Windows(R) Professional Edition, Description: Windows(R) Professional Edition, RETAIL channel, Partial Product Key: XXXXX, License Status: Licensed". Question is: can I deactivate the key on this computer, and reactivate it on my to-be new build? Also, do I need my new windows 10 product key (if there is one - can't seem to find it on my computer), or do I just use the Win 8 Pro key that I have when I reactivate windows 10 for the new build? Found an insane amount of conflicting information regarding this questions when I googled it, so I got no clue what website or article to believe. Any help is greatly appreciated!