you can apply any visa even if you already have a visa , then cancel it if you want . as you have to apply outside and online now it will be the immigration at the airport who will cancel it for you on your way back with the non-B visa
what visa you got O or B ?? he actualy talking about non-B visa mean for people work in Thailand . non-O for married , retire .... can work as well with work permit .but without taxs paid in Thailand when you got WP you can't renew it or you have good audit make fake balance sheet