yes,You'll need to buy a license key for Windows Professional, either a retail upgrade key or for the full version of Win7 Pro if you don't already have one.
You can use the anytime upgrade with the new license key.A Windows 7 Ultimate Installation can normally not change into a minor version of "Windows 7 Home Premium" or "Professional". If you have installed a preview version of Windows 7 Ultimate (e.g. beta, release candidate build 7100 or RTM build 7600) and want to install a cheaper version Windows 7 Home Premium or Professional the installation is refused.
Windows 7 comes in various editions like Ultimate, Professional, Home Premium, Home Basic and Starter. Now there might be many cases when you would like to choose the edition during setup for example, you have Windows 7 Professional key but you downloaded wrong Windows 7 Edition. So now you want to choose Professional edition during setup.