This works. Tried and tested. Simply pop the CD in under Longhorn 4008, choose to upgrade, and you're all set. It keeps nearly all document and settings intact, and does a fine job upgrading.

It clearly states in the installation that in order to create an upgrade, you must have at least 4000 or higher build number.

Though I do encourage you all to do a clean install, as upgrading is always more risky than a fresh installation.

You cannot upgrade from XP, win2k, win2k3, 98/95 or ME.

Only from Longhorn and the build has to be higher than 4000.