Last year I bought a laptop with XP Home, and installed XP Pro on top of it, with no trouble at all.
I would probably check with the manufacturer to make sure there aren't any known issues with upgrading to XP Pro. (Odds are it will be OK, I've talked to many people who have done it with no problems. Only issue I can think of is that you *could* run into some driver issues that might take awhile to resolve, but I think that's going to be the exception rather than the rule).
Kevin