Generally speaking, new motherboard = new installation, new CPU = no change unless you're going between single core and multi-core. XP, from what I remember, had single-core and muli-core versions of some files which it would automatically update if you changed CPU or enabled/disabled hyperthreading, newer versions of Windows may or may not.
Of course Windows 95/98 didn't even care if you switched motherboard, so that's been a step backwards in the last few years.
Just replaced the old cpu with the new x4 635, my ASUS board recognized a new cpu and loaded default settings, then Windows installed something to recognize the cpu. Done, all is good. No win repair/reinstall required. Refreshed Windows Experience Index, and it all shows up correctly.