Windows 7 upgrade question

Windows 7 upgrade question

If I purchase an upgrade for Windows Vista (Home Premium) to Windows 7 (Home Premium), do I retain the key and download rights to Windows 7 in case I format my computer in the distant future? Ideally I would be able to reinstall Vista again and download Windows 7 to install at any time. It seems kind of silly to me if it's just a one time upgrade, but it can't hurt to ask. Any insight is appreciated, thank you.