I just upgraded the Motherboard and processor. (Asus H87 Plus Motherboard and Intel i3 4th generation processor). normally after upgrading I used to install the operating system and all programs again. But this time windows 8 saved a lot of time.
After changing the motherboard and processor I just started the PC and it took some time to install the devices and booted normally. One thing is that you will loose the Windows activation you have done earlier as it is attached to the hardware you are using. So just activated the windows 8 again, and yes, ready to go 
I think even Windows 7 we have to install the Windows again to use it.