Installing Windows 7 upgrade version on a clean disk [duplicate]

Possible Duplicate:
Clean Install Using Windows 7 RTM Upgrade

Is it possible to install the much cheaper Windows 7 upgrade version on a clean disk? What information will I need?

1) Will the Windows 7 installer ask me for my XP license key? or 2) Will the Windows 7 installer only run if it can detect an existing XP installation?

Furthermore, what will happen if my disk crashes and I need to reinstall in the future? Will I need my XP license key again?

I haven't tested this but it looks like it can be done:

• Performing Clean Install of Windows 7 with Upgrade Disk
• How to clean install from Windows 7 Upgrade media

But whether this won't be blocked by MS at some later stage is a whole another deal.