You can use Windows Anytime Upgrade to upgrade a 32-bit version to a 32-bit version or from a 64-bit version to a 64-bit version of Windows 7. However, Windows Anytime Upgrade can't upgrade a 32-bit version to a 64-bit version of Windows 7 or a 64-bit version to a 32-bit version of Windows 7.
Windows Anytime Upgrade is
only available for online purchase in Australia, Belgium, Canada, Germany, France, Italy, Japan, the Netherlands, Spain, Sweden, Switzerland, the United Kingdom, and the United States.
Update: Windows Anytime Upgrade is no longer available for online purchase