Elemental Change permanently changes the element of the target monster. Of the 4 elements, Fire, Water, Earth, and Wind, your sage may only learn to change to one of of them. When the skill is cast, it will use one Elemental Converter of the element you've chosen. The chance of success is based on your level of that elemental endow, as well as your DEX, INT, and Job level. With medium to high INT/DEX and high level Endows, you should have close to 100% success rates.