I don't know why Technical support wouldn't help you. I had the same problem when I put in a new motherboard and also got the message That my windows 8 was used on another computer. I did call technical support and told them what my problem was. They directed my call to another party in activation and they did activate it for me. I would call them back and ask to speak to someone in the activation department and not let them direct you back to the automated way of doing it.
I did have my OS on my hard drive but apparently when you change out motherboards it isn't recognized.
Sorry didn't see where you got it fixed.