You may not have to buy Windows again. You will definitely need to reinstall because you'll need the drivers for the new mobo, chipset, etc.
I know MS uses the serial number of the CPU, hard drive, and something else I can't remember right now to develop a security key that checks that the install is a valid licensed install. After your install, it will bring something that says the current copy isn't a valid install but all you have to do is contact them and explain what you are doing and they will clear that message over the internet. I went through that the last time I had to replace a failed hard drive. That was on Win7 but they are probably doing the same thing with 8.