Have you installed it and now wanted to change ?
I think changing a installed 64 bit windows to 32 bit is not possible.
If you want to run any 32bit application on Windows 7 you can use XP Mode on Windows 7 for this.
If you install a 32bit application, it by default installs to the Program files (x86) folder on your C: drive whereas the 64bit apps install to the Program Files directory.
I cannot think of any applications that i've come across that do not work. Or when i had come across problems, you can right click the application shortcut and choose properties, then compatibility tab and tell to run in Windows XP mode (or whatever)
You will loose access to any RAM you have beyond 4GB (if you have more than that)
Most people ask how to upgrade from 32 to 64 which is not possible, so back up and restore after a clean/fresh installation.
As Nikorr and other members in this thread have stated, you cannot simply downgrade from Windows 7 64-bit to Windows 7 32-bit. When migrating from Windows 7 32-bit to Windows 7 64-bit, or vice versa in your case, it is important to note that a clean or “custom” installation will be required. For more information, see Installing and reinstalling Windows.
Also, as several members within the thread have pointed out, in most cases, applications that work within Windows 7 32-bit will also function just the same in Windows 7 64-bit. It is important to assess what architecture you will need. When considering Windows 7 32-bit or 64-bit you may find the following FAQ 32-bit and 64-bit Windows helpful. As another resource, Microsoft has provided a video outlining 32-bit and 64-bit Windows 7.
Should you encounter an application that requires a 32-bit operating system, typically those applications are known as legacy applications and will not work in either architecture of Windows 7 (32-bit or 64-bit). In this case, Windows XP Mode and Virtual PC are usually recommended tools as Windows XP Mode is a virtualized environment of Windows XP Professional 32-bit which runs directly from your Windows 7 desktop. Windows XP Mode and Windows Virtual PC are available on Windows 7 Professional, Windows 7 Enterprise and Windows 7 Ultimate. For those who are not currently running Windows 7 Professional, Windows 7 Enterprise, or Windows 7 Ultimate, there is an easy upgrade path to Windows 7 Professional or Windows 7 Ultimate through the Windows Anytime Upgrade program.
Please note, Windows XP Mode is a virtualized environment and is not recommended for graphically intense applications however, for most legacy applications it will work just fine.