You need to be very careful when you want to upgrade from a 32-bit version of Windows to a 64-bit version of Windows 8. Microsoft, for whatever reason, decided to make things complicated. If you run the Upgrade Advisor to buy a copy of Windows 8 online, you will always get the version of Windows 8 that is corresponding to the currently installed version of the operating system.
So, if you are running the Upgrade Assistant on a 32-bit version you will end up with a 32-bit Windows 8 version, while running it on a 64-bit version of Windows will result in a 64-bit version of Windows 8.
That’s not a problem if you do not want to switch from 32-bit to 64-bit during the upgrade, but if you do, you will notice that you can’t do that at all. If you are unlucky, you will notice once the system is up and running after the installation.