Got the following from MaximumPC's upgrade guide:
Can I Buy the Windows 7 Upgrade Edition For Use With Windows XP or 2000?
Yes, but there?s a catch. Only Windows Vista users will be able to do an in-place upgrade. This means that Windows XP or 2000 users will be forced to do a clean install. Maximum PC readers are generally technical enough to know that this is a good idea anyway, but if your planning on upgrading PCs for friends and family, bring a USB hard drive and be prepared to stay awhile.
This site also listed the following as related to Upgrade:
The Basics
Release Date: October 22nd 2009
Qualifying OS?s For Upgrade: Windows 2000, XP, Vista
Qualifying OS For In-Place Upgrade: Vista
Upgrade Editions: Home Premium, Professional, Ultimate
System Requirements:
? 1GHz or faster 32-bit (x86) or 64-bit (x64) processor
? 1GB RAM (32-bit) / 2GB RAM (64-bit)
? 16GB available disk space (32-bit) / 20GB (64-bit)
? DirectX 9 graphics processor with WDDM 1.0 or higher driver
My copy is due to arrive today...For anyone interested I will post results of my attempted upgrade.