I have a Win 7 pro licensed version. I wanted to make the July 29, 2016 deadline for free upgrade to Win 10. I didn’t want to loose my Win 7 configured drive yet. I purchased a inexpensive 250 Gig Solid State Drive for $65.00.
I unplugged the old drive. Plug the new one in. Put the Downloaded ISO disk of Windows 10 in the blue-ray drive. It found the drive and started the boot on the DVD. I had to partition the drive and format. During the install I was asked for the product key and I input my Win 7 pro product key. That was it.
It took about 10 minutes to install. After that I booted it booted to Win 10 and all was good.
I hope this helps someone else out there…