My eagerness to try the new Remote Desktop app made me make a huge mistake

I found out you need a Pro version of Windows to use Remote Desktop. I then remembered I had a picture of a W8 Pro key on my OneDrive which I bought back when it was on sale. I had already used the key on my old W7 laptop but I assumed it would be deactivated on that when I activated it on a new machine. I went to the "add features to Windows" and typed the key in. Everything seemed to be fine. It accepted it and I could turn on remote desktop, but then I entered the settings and saw to my surprise "Activate Windows". I then reentered the key and it said it was already in use. Why did it accept it in the first place if it's not valid!? I then deactivated it on my old laptop, but when I tried to activate it on my new it activated it on the old one again (WTF?). I just read that W8 and W8.1 keys are different and therefore I can't use it on my new W8.1 laptop. Now to the question: Is there anyway to back to the my old regular W8.1? The laptop (Samsung NP770z5e) came with W8 preinstalled without a disc or key. There is no way I will pay 370 dollars for a Pro key when my regular W8 was genuine and I just want to have it back.

Tl;dr: I tried to upgrade from W8.1 to W8 Pro (thought it would become W8.1 Pro) with a used key and am now stuck on a non-activated W8.1 Pro computer which sucks. Help me get back to W8.1.

Thanks for your help Tribe!