Hi,
Please tell me whether I can upgrade from Windows 7 32 bit to Windows 7 64 bit without doing a clean install (just with the upgrade option).
No. If you are currently running a 32-bit version of Windows, you can only perform an upgrade to another 32-bit version of Windows. Similarly, if you are running a 64-bit version of Windows, you can only perform an upgrade to another 64-bit version of Windows.
If you want to move from a 32-bit version of Windows to a 64-bit version of Windows Vista or vice versa, you'll need to back up your files and then choose the Custom option during Windows Vista installation. Then, you'll need to restore your files and reinstall your programs.
source: 32-bit and 64-bit Windows: frequently asked questions