As published in FAQs of Windows 7 by Microsoft on their website,
Can I upgrade from a 32-bit version of Windows to a 64-bit version of Windows 7 or from a 64-bit version of Windows 7 to a 32-bit version of Windows?
You can use the Upgrade option during Windows 7 installation, which preserves your files, settings, and programs, only if you’re currently running a 32-bit version of Windows Vista and you want to upgrade to the 32-bit version of Windows 7. Similarly, if you are running a 64-bit version of Windows Vista, you can only perform an upgrade to a 64-bit version of Windows 7. For more information, go to Upgrading to Windows 7: frequently asked questions on the Windows website.
If you want to move from a 32-bit version of Windows to a 64-bit version of Windows 7 or vice versa, you’ll need to back up your files and choose the Custom option during Windows 7 installation. Then, you’ll need to restore your files and reinstall your programs. For more information about performing a custom installation, see Installing and reinstalling Windows 7.
See here: http://windows.microsoft.com/en-us/windows7/32-bit-and-64-bit-windows-frequently-asked-questions
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