|Did you already have Win7 32-bit before you upgraded the RAM to 8GB, or did you already have the 8GB RAM when you installed Win7 32-bit? Just curious...|
As OtheHill stated, you will have to do a fresh install of Win7 64-bit.
EDIT: I missed this - "want to go back to 64". Apparently you already had the 64-bit version installed? Why did you replace it with 32-bit?