Now that Windows Vista has officially launched some consumer scrutiny has provided some invaluable insight into the new OS's EULA.The document, available from Microsoft's website, clearly states that any user upgrading to Vista from XP will no longer be able to use the upgraded copy of XP.
The Vista EULA, suggests that once a consumer has purchased the upgrade he will not be given the option to revert to XP in case, for example, he does not like Vista. The relevant EULA article states, 13. UPGRADES. To use upgrade software, you must first be licensed for the software that is eligible for the upgrade. Upon upgrade, this agreement takes the place of the agreement for the software you upgraded from. After you upgrade, you may no longer use the software you upgraded from.
Considering that upgrades will be a popular Vista migration option it is important that users realise how final a commitment their upgrade will be. Most XP owners are likely to wish to wait until a verdict on Vista is out before rushing to upgrade.