Via is a company that makes chipsets, just like Intel, Nvidia, SiS, etc. If you have a 945 (Intel) board and switch to a P35 (Intel), you shouldn't have to reformat. Before you switch over, go to the device manager and remove the drivers for your NIC, Sound, Video, etc. All the stuff thats on the motherboard. Move the hardware over, load windows. It will load very slowly. It will install what drivers it has, but you'll probably have to use the driver disk that came with the board. Once thats done, you should be done.
+1. OP should not have issues booting. But OP may need to boot in to Safe Mode for Windows to load properly and will have to install drivers in Safe Mode.