There are several important things to bear in mind when changing a tow ball coupling. First and foremost you must know the total weight of your trailer. If you are only changing the tow ball coupling, you should change to a tow ball coupling that is designed for the same total weight of your trailer, or more.
Then the location of the holes is important; the two most usual types of location are straight and diagonal, see the illustrations. Measure the distance CC between the hole centres as shown in the illustrations.
Finally measure the diameter of the coupling tube. If the tow ball coupling does not fit, it is often possible to use spacers.