I've got several copies of XP Pro on the go, I've reactivated quite a few times after upgrades (one is the kids' PC, when I upgrade my main PC with a new CPU, for example, I'll pass my old one down the chain to upgrade their machine). MS have never quibbled about it when I've phoned for a new code.
The only hassle I've really had is that getting my ethernet and wifi setup sorted last year saw me swapping things around a bit to get it all to work, and that had a tendency to trigger the code request....
Dave