Re-using a OEM Windows copy on another machine?

Hey, apologies if I don't know how to explain the problem real well, bit of an unusual case.

I've recently put together a PC for a family member, and thought instead of buying another copy of Windows outright, I thought I'de try to use an OEM copy of Windows 7 that I updated to Windows 10 for free with Microsoft's offer a while back. Around a year or two ago I had my local computer shop upgrade my PC and at the time they 'needed' to get a new copy that wasn't an OEM and can be re-installed easily. I guess what I'm asking is can I use the key from the OEM copy (it is visible in my Microsoft account) on this new PC? I already have bootable media for Windows 10. Any help on this would be appreciated, please let me know if this crosses any moral 'grey-areas'.

*EDIT* I know typically OEM copies cannot be re-used (in the cases of Windows 7/8/8.1 OEMs) but I was hoping Windows 10 had somewhat rectified this issue? I haven't really found anything on this anywhere. The motherboard used in the build is the original motherboard the OEM license was used with, but it does not have the same storage drives, I still have the key as well.

It sounds to me like you have 1 legitimate license for Windows which means it can legally be installed on only one computer. Using 1 license for Windows to have it installed on two computers at the same time is illegal. You would have to remove it from your computer first.

My current PC, which was upgraded with a new motherboard, cpu, the works is on a newer copy of Windows that isn't OEM, but is legitimate, and the OEM copy that was hashed to my old motherboard, which is what I am using in the PC I've built is also a legitimate copy, if I were going to pirate it I wouldn't be here. After some more research I found there were changes to the activation/validation process in Windows 10. I was asking if anyone had any clue if I could reinstall Windows with slightly different hardware but the same motherboard. I do have both the key and bootable media from the Microsoft website.

Also note it has been around 9-12 months since this OEM copy was used.

Ok, thanks for all the replies, once I've got the bootable media fully downloaded and loaded onto the USB, I'll give it a go. I'll post the results here.

If you have a problem then phone them up. You can get the phone number via typing SLUI 0x4 from a command line. Choose the country you want for a local call in the language you speak and be prepared to be on hold for an hour or more.

As long as you aren't taking the piss (and let them sign on) Microsoft will activate your install with a new Windows 10 retail key.

You should then use ShowKeyPlus (see above) and write it down just in case.

Do you have 2 product keys for Windows 7 or 1 each for 7 and 10? I would try the Windows 7 on the new computer first and if that doesn't work then the Windows 10 product key. Then whatever product key is left over will be the license for your computer.

Your activation will not be affected because you already have a permanent digital license that activates your computer.