hmm maar deze site zegt dat het wel kan
Windows 8: upgrade from 32-bit to 64-bit system
In short: you can’t use the Upgrade Assistant to upgrade from a 32-bit version of Windows to a 64-bit version of Windows 8.
It won’t work if you run the installation directly using the Upgrade Assistant, and it won’t work if you select to burn the operating system to DVD or copy it on an USB Flash Drive instead.
If you have the option, you can run the Upgrade Assistant on a 64-bit version of Windows and burn the contents to DVD or copy it on a flash drive. If you do not have the option available to you, you need to buy a 64-bit retail upgrade copy of Windows 8 instead or select the backup media add-on option during the purchase which also includes two discs. The retail copy includes two discs, one for 32-bit and one for 64-bit.
I’m not sure why Microsoft is making it that difficult for customers who try to upgrade from 32-bit to 64-bit during the upgrade. It certainly would not be a problem for the company to add an advanced configuration to the Upgrade Assistant that can be used to pick the version of Windows you want to upgrade to.
Microsoft notes that you can’t keep any files, settings or software programs when you upgrade from 32-bit to 64-bit.
En deze site ook
http://www.winsupersite.com/article/windows8/windows-8-upgrade-32bit-64bit-144649
Upgrade from Windows XP/Vista/7 32-bit to Windows 8 64-bit
So wait a second. Is there a way to actually upgrade from a 32-bit version of Windows to a 64-bit version of Windows 8?
Yes.
Yes, there is. But you can’t do it electronically. Instead, you will need to purchase the retail Windows 8 Pro Upgrade media, which is currently $30 more expensive (or $69.99) than the electronic upgrade. This package provides both 32-bit and 64-bit media, so you can choose which to use. (In case this isn’t obvious, you can’t actually do an in-place upgrade from a 32-bit version of Windows to a 64-bit version fo Windows 8. Only clean installs (“Custom” install type) are supported.