You only need to transfer the visa once to your new passport. If you get a new passport while outside Thailand, they'll do it when you fly in. Otherwise, go to an immigration office and they'll transfer the visa over to your new passport.
14 days is for visa on arrivals, those who aren't eligible for visa exempt entry such as Chinese or Indian passport holders. 14 day visa on arrivals are not extendable and not able to be changed to another visa in country.