The upgrade should work as it's 64 bit to 64 bit. You cannot upgrade from 32
bit to 64 bit or vice versa. I have seen some Vista Home Premium to Vista
Ultimate upgrades fail on computers with an OEM version of Vista. In most
cases the problem was the AV/Firewall/Anti-spyware program that came with
the computer. It may need to be uninstalled before the upgrade will work.
The AV program would not allow some needed registry changes. Note that most
OEM computers do not come with disks to reinstall the AV. If you uninstall
the AV you may not have the means to reinstall it.
In other cases the problem was a incompatible OEM boot sector. The boot
sector was non-standard to support their factory restore procedure.
Overwriting the boot sector with a standard Vista boot sector allowed the
upgrade but it broke the factory restore procedure. Before you do this you
need to make sure you have created an alternate method to restore the
computer to it's factory setup if need be. You will also need a Vista DVD to
boot from. You use the bootrec /fixboot and bootrec /fixmbr commands. Se the
following link for details.
http://support.microsoft.com/kb/927392
Note that this may disable the OEM factory restore procedure so proceed
carefully only after you have a way to restore the computer to the factory
setup. You may want to sell the computer some day in which case you would
need to restore the factory setup.