littleleo :
Per Microsoft you can not upgrade or downgrade a system form 32bit to 64bit or 64bit to 32bit. You will have to get a Windows 7 32bit version and do a clean install.
Not exactly.^
You can freely change to/from 32 and 64 bit versions of windows without needing to buy a new license.
What they mean is that you need to do a clean install, not an upgrade install that preserves your apps.
To do this, you need a windows dvd of the appropriate 32/64 bit version to do a clean install.
You can borrow the dvd to make the change if you do not have one.
The activation code will be the same as the original 64 bit install.
One reason for 32 bit:
I have changed a laptop from 64 bit to 32 bit for the express purpose of being able to play civilization 2.42 while travelling
old 16 bit programs run on 32 bit windows but not 64 bit.