Given an IFeatureLayer, "theLayer", one would cast to a IGeoFeatureLayer, then get its IFeatureRenderer and cast that to a IClassBreaksRenderer. Then it is a s imple matter to set the IClassBreaksRenderer's Field to the field to be rendered.
IGeoFeatureLayer lyr = theLayer as IGeoFeatureLayer;
IFeatureRenderer ifr = lyr.Renderer;
IClassBreaksRenderer cbr = ifr as IClassBreaksRenderer;
cbr.Field = "someFieldName";