Strugs99,
The path you describe is actually bringing a smile to Microsoft shareholders all over the world, as such a path would indicate a customer who has been with the Company for almost 10 years, if not longer, and who wants to continue to patronize Microsoft and purchase its products.
Your question about the motherboard change would be relevant only if the license in question were an OEM license. If the licenses you mentioned are retail upgrades, then the question of motherboard is not going to come into the picture. The Vista Upgrade license requires that a previous qualifying Operating System be installed on the computer, so if the Vista Upgrade DVD runs happily on the installation of XP you have, and does complete the upgrade from your existing XP to the new Vista, by definition you have a legitimate installation of Vista.