Windows 8 64-bit Upgrade from Vista 32-bit

I only had the 32-bit version of Windows Vista installed on my laptop so the Windows 8 upgrade assistant detected this and only downloaded the 32-bit ISO for me, however I wanted to install the 64-bit version. Here's how I did it:

I downloaded a Windows 7 64-bit ISO from this site, and installed it inside a Virtualbox VM (I just clicked next when it asked for a product key and it let me pass). Then I downloaded the Windows 8 Upgrade Assistant from within the VM and ran it, it will then detect your running on a 64-bit OS and will download the 64-bit version of the ISO for you. I then burnt the ISO to a DVD and used that to upgrade my copy of Windows Vista.

During the Windows 8 install process I formatted my target drive as I wanted a fresh install and it didn't seem to have a problem with this, I assume it had already detected a valid OS to satisfy the upgrade. If you don't already have a valid OS installed for the upgrade it appears you can still use the upgrade ISO according to this guide.