G'day 2cats.
Usually, if it's been longer than ~90 days since you last activated & you've only activated that key a few times then it should reactivate online no probs.
Worst case scenario is a free call to MS, say you've changed motherboards & then still no problems.
I've reinstalled/activated a version of XP I have no less than 12 times. I'm not sure what the actual number of activations allowed b4 needing to phone MS is but even if I let it go for more than 90 days now I still need to make that phone call. I think it was after ~4 installs.
Either way, it's never a problem just a hassle.
I tend to agree with Ice. You could get away with a repair install & loading the new mobo' drivers but issues can arise. A fresh install is always preferable
Cheers,
Ruffy