What I would suggest...
If just replacing the CPU and the motherboard remains the same, no need to reinstall the OS.
If changing the chipset (motherboard), better to do a fresh install, else reinstall the respective chipset drivers.
A fresh install always 'recharges' the system, and makes it quicker and efficient. You could say 'unbloats' it. Though the flip-side is you will need to install all your applications again, pull the M$ updates.