Upgrading from Vista 32 to 64

Hey everyone, I have an OEM copy of Windows Vista 32bit intalled on my desktop... would I be able to upgrade to Windows Vista 64bit by downloading an upgrade or would I have to purchase a copy (OEM or Retail?) of Vista 64, uninstall Vista 32 and then install Vista 64? How can I do this? Thanks.

I am not 100% sure about OEM. I do know that the retail versions had details on how to contact Microsoft and they would ship a 64bit version for basically a shipping fee. The only exception is Ultimate, which ships with both 32 and 64 bit disks.