XP Pro from Home is a legitimate upgrade. You will not need to reformat and just need the XP Pro upgrade version disk. MS has a grid (sorry, no link) showing which products qualify for an XP Home or Pro upgrade. In the event you ever need to reformat and rebuild your PC, you will need both the Home and Pro disks so don't get rid of the old one. What you will get from Pro vs Home will largely be security features. If you are not looking for such, Pro will probably be of no value. Hope this helps.
I would like to upgrade my PC from XP Home to XP Professional. Has anyone done this? Can I just do an upgrade? I don't want to format and do a fresh install since I have so much software already installed. Any advice?