with a new mobo and cpu you will have to do a fresh install, because your new motherboard will require different drivers for a different chipset etc etc
no need to update to windows8.1 unless you want to. you can reinstall windows 7 using the same serial if windows will not activate on install , call the microsoft number on activation and they will give you the required codes to activate
I would recommend a clean install of Win7 - typically, Windows 7 can find a lot of drivers but it's best to grab the chipset and network drivers from the motherboard manufacturer's site ahead of time and stick them on a flash drive. Then, once you boot into windows install the chipset and network drivers and let windows update hunt for the rest. If you find any devices that haven't popped up, you might have to do some guess and check work because sometimes the descriptions in device manager can be a little non-descript. Nonetheless, I think that's the best way to go.