I assume you're planning to use your current SSD containing Win 8.1 OS as your new boot drive with the new motherboard.
If you're basically satisfied with the operation of the current system on the current drive, i.e., it boots & functions without any problems and you're planning to continue using the Win 8.1 OS, then I would see no need for a fresh-install of that OS onto the SSD.
Now I assume you understand that there could be a licensing issue here involving the new motherboard depending upon the type of Microsoft Win 8.1 OS installation media you originally purchased. So if you're unfamiliar with that potential issue, do some research on it.
And you do understand that there's a likelihood that you will need to access the motherboard's driver installation CD to install any necessary drivers required by the motherboard's system, right?