Another consideration is that your pond should have some freeboard between the normal pool level and the elevation of the emergency spillway (or the top of your embankment if there is no emergency outlet).

If you install a new overflow tube and raise the level by 3', then a big rain could go over your dam while the 6" drain is not keeping up with the inflow. That could wipe out your dam and fish population.

There may be a very good reason for the pipe elevation they selected in the original pond design.