Yeah, I know a clean install is "best," but I think I'll give the upgrade/repair route a try. At least under XP and 2000, Microsoft noted you could do this via an upgrade from within the OS and then, on the first reboot, turn off the computer, replace the motherboard, and then continue (or do a "repair" booting from the OS disk with the new motherboard). See
How to replace the motherboard on a computer that is running Windows Server 2003, Windows XP, or Windows 2000. This reinstalls the HAL and other drivers appropriate for the new motherboard. It isn't clear to me that Vista operates, for this purpose, any differently--and I'm game to give it a try. (I'm not certain whether setting the "detect HAL" option in msconfig adds anything to that or not.) As for old, unneeded drivers, some may be able to be removed manually.
At least, I'm going to give it a try. The worst that happens (I hope) is that I waste some time before doing a clean install and reloading all the programs.... It will be a couple of days before I get around to it; I'll post how it goes.
Thanks.