The correct upgrade method to a higher version of Windows 7 is using Windows Anytime Upgrade - How to - Windows 7 Forums,
which will accept a retail Professional key of the same bit-version even though an Anytime Upgrade key is what's intended and much cheaper.
However the workaround Shawn provided is a convenience for using retail Upgrade when the HD is new or wiped and no other OS can be seen by the booted installer to allow use of Upgrade version key.
Since you have an installation now I'd just run the Option Four Upgrade over itself as suggested.
Be sure to save a Windows 7 backup image of the completed install once it's set up and running as you wish, to use in place of reinstalls.