Upgrading from 32-bit to 64-bit Windows Vista

We had the first of our 'Ask the Expert' sessions this morning (you can still register for the others, here!) and there were a load of great questions - some even had us stumped!
One of the questions that we were asked was around upgrading from Windows XP to Windows Vista Business, but on different architecture, so from x86 (32-bit) to x64 (64-bit). Initially, I thought this couldn't be done, and would require a clean wipe, and a reinstall with the full Vista version rather than the upgrade version. I was a little bit wrong...
This KB Article: Installation choices for 64-bit consumer versions of Windows Vista details the ways you can upgrade from x86 to x64, in a number of different scenarios. In response to the actual question, to go from x86 XP to x64 Vista Business, you need to do the following, but before this, there are a few pre-requisites:
You need to have purchased the Windows Vista Upgrade (Or Full version) - in this case, we'll be using Vista Business 32-bit Upgrade. You will then have to access the 64-bit version (unless you are running Ultimate, as this ships in the box) from here.
After you get hold of the editions, x86 and x64, follow these steps:
Back up all the data and the settings by using Windows XP Backup.
Note You can also use Windows Easy Transfer. This back up program is available on the Windows Vista DVD. However, you must use the version that is on the 32-bit Windows Vista DVD if you want to use Windows Easy Transfer to back up a 32-bit version of Windows.
Insert the 64-bit version of Windows Vista into the system DVD drive, and then restart the computer.
Start Windows Vista Setup from the DVD when you are prompted. Note You must start Windows Vista Setup by starting the computer from the Windows Vista 64-bit DVD. The installation package will not run on a 32-bit operating system.
When you are prompted during Windows Vista Setup, remember to select Custom as the installation type.
After installation is complete, you can restore the data from its backup location.
OK, so that was pretty straightforward - if you want any further info on 32-bit upgrades on their own, you can read this KB Article here: Installation choices for consumer versions of Windows Vista (32-bit only)