Larsenex just use the product key you had for windows 7 for the windows 10 installation.
Everything should go fine as long as microsoft activation servers have it on file that its legit and registered.
Your not going to like what I am going to recommend but use the windows 10 media creation tool and make a bootable usb drive.
Then you start it up from wiithin windows 7 and have it reboot, it will kick on to the windows 10 installation process.
Find your windows 7 partition and delete it, let the windows 10 installation do a full format and let it go from there.
Thats called a clean install so you have no other older windows installation in the way incase something conflicts over time like drivers and registry messes.