Funny, I recently replaced the mainboard for the p8p67. Former board was asus p5qc.
Now due to a cpu that appeared to run hot in bios, I decided to boot from the sata drive with win7 on from the previous board and check with some programs for temps and voltages etc.. After some struggling I got to the desktop, eventually most new hardware adjusted and asked for new drivers etc.
But it leaves quite a mess on your system in this case. Needless to say when I found all hardware appeared to be installed correctly I did a clean install.
As said by others, replacing the cpu requires no reinstall or anything similar.
You can run it with the new speed without a problem.
It's only with videocards problems could arise, mainly because of drivers that tend to stick, or create conflicts like when going from nvidia to ati/amd or viceversa.
And ofcourse mainboards. Due to the different chipsets etc.