If you have upgraded from a previous version or followed the steps here in securing the GenuineTicket.xml file and tucking that away on a removable or other safe place and plan a complete clean install you would need to buy a key. Changing over from the 32bit to 64bit once 10 has been activated won't null the record of the activation from the previous 32bit install. That simply gets transferred over to the fresh 64bit.
Here I saw the second clean install for the 64bit on the laptop go and activate following the first when I found I had to nuke the entire drive since the factory partition could not be resized to see a second backup partition created. Once the drive was wiped that second 64bit install activated on the spot since the record of the initial upgrade was kept by MS. The hardware profile ID remains the same just as if you had bought 7 retail years back and had both dvds for the 32bit and 64bit. You could easily wipe the 32bit off at any time for the 64bit or go back to the 32bit after putting the 64bit on. Likewise once the machine has been activated the first time you can reinstall the same version/edition of Windows an endless number of time for either kernel.
As far as the article there that only applies if this guide hadn't been posted or simply if no one had found out the option to avoid he need for the upgrade install first in order to see 10 activated which is what this guide explains. In fact once you have the xml file from the previous version you should be able to simply slap the 64bit 10 and not have a worry since that doesn't determine whether or not 10 or any other previous version gets activated.
The method outlined in the guide sees the previous version's activated status transferred to the fresh 10 install regardless of it being 32bit or 64bit since you are not performing a direct upgrade type of install. The record of the activation isn't stored locally on your drive but on the MS servers making that possible!
In fact that's how they keep track. Software companies also do the same and require you to uninstall their programs while connected online so their own server keeps a record of the uninstall while they only a few reinstalls to be seen before charging you again! That's another big difference in how MS is handling their activation policies.