MS has said that the 32 and 64 bit versions are interchangeable. so long as you have a valid key for win 7, then either the 32 or 64 bit version can be installed.
you will have to do a fresh install though so back up anything you want to save. there is too much different under the hood to pretend to do it any other way. wipe the 32 bit install and then do a fresh one for the 64 bit install. should work easily. i have done this a few times myself with success.
So, have i good configuration of my computer parts to install Windows7-64bit???