Replaced Motherboard Deactivated Windows 10 Pro

[Llorty]

Due to an the insider build 10130 not updating at all I did a clean install of windows 10 which fixed it, but meant my windows 7 cd key was gone.
My desktop's power supply malfunctioned last week taking out with it the GPU and Mother board and all three where replaced
When I got my desktop back it said that I had to activate windows so I tried to activate it through settings with no results. So I rang when I got the PC from and they said that the windows activation is tied to the motherboard, which has just been replaced. They told me to do slui 4 which I did and call microsoft if I had any trouble
I tried 4 times and of the 4th time I decided to go with the prompt at the end and call Microsoft.
The first person said that I had to buy a new copy of windows 10 or speak to the store I got it from
So after getting pricing for a new copy of windows 7,8 and 10 from the store, I rang up microsoft to ask if my old windows 7 cd key would work again to avoid the trouble of uninstalling windows 10, reinstalling 7 and upgrading back to 10 and he didn't listen to the fact that my motherboard had been replaced and said it will be okay in 24 hours due to the servers being under heavy load
However I read that the insider activation route is tied to the mother board.
What is the best way to go about activating windows 10 again?
My old cd key is 7 home
and my current (genuine) version is windows 10 pro.

*Edit

I just installed windows 7 again and my original product key is no longer active

 

[James Mason]

You can't go from a home version to Pro, that's probably the issue.
You need to use a regular windows 10.
If you still have your CD key, you can reinstall 7, and then upgrade to W10 (home/not pro)
Or you can buy Windows 10 Home/Pro and clean install it with the new key you get when buyign it.
The windows 7 product key would be on a sticker on the case if you bought the PC, or would be with your windows retail/oem material assuming you kept it which you should have because it has your key.

You could alsojust buy a copy of windows 7/8 again and then do another upgrade.

You can't do a "clean install" without doing an upgrade first, unless you outright bought windows 10.