The only reasons I can think of to warrant an upgrade from Home to Pro or Corporate is the advanced networking to large domains etc. and the encryption feature.
If you were going to do it, then I would backup your most important files and do a clean install (i.e. format the drive or partition and install fresh).
Otherwise it is totally possible using a copy of XP Pro and running it from within Windows.
There is a chance, that if your XP Pro disc is SP1 or less you may have to do a 'slipstream' to incorporate the new service pack into a new disc.
I wouldn't worry about that though unless you come to it. If you do there is help here:http://www.winsupersite.com/showcase/windowsxp_sp2_slipstream.asp
Good luck 69ers