The only way it will ask you for a serial it's if you clean install Windows 10 on your computer. Which you can do, without installing Windows 7. You clean install and it prompts you twice about a serial which is the one you can skip. And when windows 10 is installed it will detect your hardware it will activated because you were on windows 10 before. Because if you are upgrading from 7 to 10 it shouldn't ask for a serial and it should only verify you have a valid Windows 7 serial you can upgrade to Windows 10.