Thanks for this Frank!
I was having trouble changing the legend label (override 1) generated by the print task for a jimu/DrawBox graphic which is being used to show the 'area of interest' in the map.
If anyone else has trouble modifying the legend for DrawBox layers, here is the code I used in the "draw-end" event of the drawBox:
      var simpleJson = {
        "type":"simple",
        "label": "Area de Interés",
        "symbol":{
          "color": [
            0,
            112,
            255,
            64
          ],
          "outline": {
            "color": [
              0,
              0,
              0,
              255
            ],
            "width": 1,
            "type": "esriSLS",
            "style": "esriSLSSolid"
          },
          "type": "esriSFS",
          "style": "esriSFSSolid"
        }
      };
      var drawboxRenderer = new SimpleRenderer(simpleJson);
      this.drawBox.drawLayer.graphics[0].symbol = null;
      this.drawBox.drawLayer.graphics[0]._graphicsLayer.setRenderer(drawboxRenderer);
      this.drawBox.drawLayer.graphics[0]._layer.setRenderer(drawboxRenderer);
Hopefully this might be helpful for somebody,
Cheers,
Joe