You will need to reinstall the OS if you change the motherboard, and reactivate windows. Be careful if you do this, microsoft do not like you changing motherboard and the OEM licensing agreement expressly forbids what you are planning to do as they say that the computer has completely changed (you are allowed to replace motheboards with equivalent by same manufacturer only). If you had an identical computer, you could perform a sysprep on your disk to make it a general PC disk and reinstall the required drivers for the new motherboard and processor.
Whatever you do, I suggest that you revalidate windows by calling them, it's normally automated and will be less likely to pick up hardware changes.