Actually there's a way to swap motherboards without having to clean-install Windows.
Before doing the swap, open Device Manager and add the "Plug-and-Play BIOS (fail-safe)" driver, then remove ALL other items. And I do mean ALL other items - remove every hardware device on the system. Then shut down and swap boards. When you restart the PnP BIOS driver will start detecting new hardware. You'll need to install chipset drivers a.s.a.p. (VIA 4-in-1, etc.) to get all of the hardware detected but it'll do it.
O d d O n e, who went form a BE6-2 to a P3V4X that way - safely.