Theoretically what you could do is; download the ISO of your version of windows 7 and burn it to a DVD. Then use that to do a clean install with your old key. I'm not sure how this would work with the new UEFI BIOS, seeing how the Key is embedded into the UEFI.
Neither Microsoft nor legitimate retail stores will sell you just a license. You will get both the product key and the installation software for one price.
Well then that answers my question. To get a new license I buy another OEM. Simple.
Yes. Buy a new Win 7 with new product key and installation software.