The Chasam Sofer in Toras Moshe Parsha Emor writes the following:
A cohen should marry a basuleh (virgin). The Ramabm counts this as a Mitzvas aseh (positive commandment). However one could have the following doubt: If the Cohen Gadol took a basuleh when he was a Cohen hediyot and then became a Cohen Gadol, would he now be required to take another basuleh (get married again)? If so, since (according to the Rambam) a Cohen Gadol is not allowed to have two wives, he would have to divorce the first wife.
Why does the Chasam Sofer have this doubt that the Cohen Gadol should then get married again? Meaning if the Cohen Gadol would want to get married he needs to marry a basuleh. However in this case that he's already married, why should davka get married again when he becomes the Cohen Gadol?