The only thing that "must" be replaced is the gasket. If you are really careful and can keep it intact you can reuse it but since it holds the oil in the motor its worth replacing. If you do not plan to drain the oil you can tilt the bike to the right side to keep the oil from rushing out. One other thing that might be worth doing while you have it apart are the seals from the sprocket shaft. There are two of them, one behind the sprocket and the other on the cover itself. If the bike has miles on it i would change them too. Other than that its pretty straight forward.