format after installing a new cpu?

do you have to format/reinstall windows if u swap out JUST a cpu?

nop, you don't need to when you change any of the hardware, but i usually do it when i change the board to make sure all old chipset drivers etc are clean, but for cpu no, not needed

Nope.

If you change motherboards and it is a different chipset, a format could be needed. Instead you could sysprep the install to prepare it, or manually remove the chipset driver.

For the processor, it is just a drop in upgrade with nothing else necessary. Depending on your motherboard, less often, you may need a BIOS update to support newer processors properly.

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.