Replacing union fittings of a bathtub will not necessarily prevent or fix a leak. You must first determine if that is really the source of the leak. If you have this confirmed, you must then check if the pipes being connected by the union fitting are in a straight line. If not, that will mean that there was a gap between the fitting and the pipes, which means the fitting may not necessarily be the problem. Next, determine whether the pipes being held together are free from dirt and debris since these can obstruct a good and tight connection. If you have checked these things, replace the union fitting, and the leak should stop.