A parallel-plate capacitor is connected to a 12 V battery an…

A parallel-plate capacitor is connected to a 12 V battery and is allowed to fully charge until it is storing a certain amount of energy, U.  We then remove the 12 V battery and replace it with a 24 V battery.  We make one more change to the capacitor so that that it once again stores an amount of energy equal to U.  What change might we have made?