Perhaps your XPH with SP2 is considered newer then an XPP without SP2.
What's the advantage for you in upgrading to Pro and why not do a clean install instead?
I am running Windows XP Home edition and I am completely up to date (SP2 installed). However, I now want to upgrade to Windows XP Professional. When I try to install it, it tells me I am installing an old version overtop a newer version (I guess it considers my installed Windows XP Home the "newer" version). I am wondering if there is a downside to completing the upgrade (I didn't do it once I saw that message). Anyone know?