First, MS wants you to use Anytime Upgrade to upgrade within Windows 7. Any upgrade using the retail installer thus needs to be a clean install or it will fail with error to use Anytime.
If you are asking whether you can change your qualifying OS to XP while keeping in place your existing 7 Home Premium for the purpose of upgrading to 7 Professional, I believe it can be done with the popular workaround used by many beta testers to change to their retail version of Windows 7.
No keys of qualifying XP/Vista are ever blocked in the first place, nor is Windows 7 a qualifying OS for Windows 7 Upgrade anyway except using Anytime. These are merely EULA provisions which you want to correctly comply with by using XP to upgrade to Windows 7.
The workaround tricks the installer into thinking you are doing a same-version Repair Install
which is always allowed and which resets Activation. Since it is an Upgrade install, it should take an Upgrade version key.
If you want to try this, first save a Windows 7 backup image of your current install so you can start over if necessary: Backup Complete Computer - Create an Image Backup - Windows 7 Forums.URL
Next set a Restore Point, then change the Registry keys shown in the screenshot here to the exact name of the version you are Upgrading to:
Now run the Upgrade installer for the higher version from your desktop. Afterward activate at Computer>Properties link. If it refuses key, use the registry workaround given here: Clean Install with a Upgrade Windows 7 Version - Windows 7 Forums.URL