Do I need to format

[darkmos]

I would like to know if I am going to format my OS sdd when I change from asus amd motherbroad phenom 9950 cpu to asus rog hero z97 motherbroad and I5-4670k. 

[joshuamurphy]

Yes, while you can get away with not formatting you OS, it is possible that you will get a BSOD on boot.

[darkmos]

thanks for the answer

[Yummychickenblue]

I think there are ways around it like uninstalling the motherboard/CPU drivers and than entering the fancy commands in the command prompt that Microsoft doesn't want you to know about that disable your windows key but I'm not sure.

[LogicalDrm]

Its highly recommended but not necessary. Since you are going from AMD chipset to Intel, its more likely that you'll need to reinstall OS.