When you boot the Windows 7 DVD to install Upgrade Version it scans the HD to see any prior OS to allow use of Upgrade version key (even if you go on to delete, reparition or format).
You can install Upgrade version to an empty HD by not installing the Product Key upfront, then doing one of the workarounds in this tutorial after install: Clean Install with a Upgrade Windows 7 Version
You do not need to have the prior qualifying OS installed to use Upgrade version. Under the EULA you only need to have it on hand and not use it again once Windows 7 is installed.
A retail Upgrade can migrate to any computer of your choice for life, as long as it is on only one at a time.