Change Beam System Levels

When you select a beam that's part of a beam system or if you select the beam system itself; the Reference Level and Work Plane option are grayed out.

To remedy this:

Once the beam is selected, click the edit workplane button on the ribbon, then specify your new workplane by choosing a level from the pulldown or selecting a line from a view.  I haven't used the last two options so Im not sure what they do :)