First of all, why do you "have to do this"? What functionality do you think pro will give you that you don't have now?
Upgrading should be fairly straightforward - but, it's not always. Thats why you should have your data (not programs) backed up, and installation disks for any software that you may need to reinstall. Odds are you won;t need it, but you might.
Personally, I'd backup everything, make certain I had the installable copies of everything, get (in advance) any device specific drivers I might need (video, network, etc) and have them on a CD, just in case. Then I'd try to do an "upgrade" installation, but be prepared to reformat the hard drive and start from a clean disk, just in case. You may be one of the 98%, or you may be one of the 2%. Feeling lucky?