I am not sure I agree it is that simple. A quad to an 8 core? What 8 core, or more importantly, what motherboard?
Understand the motherboard is considered the heart and soul of the computer and as such, replacing a motherboard (with anything other than an "exact" replacement during a repair action) is considered an "upgrade". And for licensing purposes, an upgraded motherboard is considered a new computer. And a new computer requires a new Windows license. The exception is if your current license is a full, boxed "Retail" license - NOT an OEM license that came with, or was purchased for another computer or motherboard.
If you are using the same motherboard, then I don't believe you will need to reinstall Windows. I have never had to reinstall the OS when upgrading to a new, supported CPU. I have had to, however, upgrade the BIOS first, to add support for that new CPU.