It is not necessary to do so. You should look at your problem first. Run some tests or look at which ball joint is creating the problem. If it’s only the upper joint, then you should replace only the upper joint, and if it’s the lower one, then don’t need to change the upper one. But you can change both at the same time. It depends on your choice and price. If the price is high, changing both is an unnecessary expense. But you can do that if you don’t care about the cost.