If you're installing a new motherboard you will want to reinstall windows. It's not a good practice to leave an existing installation of windows after making a major hardware change. Starting with a fresh install will ensure no troubles due to leftover bits and pieces or driver conflicts from the previous installation occurs.
You would need to create installation media and you need a product key for this. If your system came with a sticker that has a product key on it you're ok, if not there are utilities that will help you extract the product key from the registry. You can find information for automatically creating windows 8 and windows 8.1 installation media by googling it. Same goes for the product key utility.