Upgrades de arquitetura cruzada não são possíveis - não foi assim com o Windows 7 e o mesmo com o Windows 8.
Users will be able to upgrade to Windows 8 from Windows Vista (without SP1 installed) but only personal files (meaning data only) will be maintained. If upgrading from Vista with SP1, personal data and system settings will be maintained.
Microsoft also is not allowing users interested in doing a cross-architecture -- i.e., 32-bit to 64-bit -- install to do so. Whether running Vista or Windows 7, these users won't be able to keep their existing Windows settings, personal files, and applications or data. They won't be allowed to upgrade this way, period.