Upgrade Longhorn 4008 to 4015
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.