title: Change valueField config on Ext-JS combobox?
body: <p>I have a combobox on a form where I need to reset its store along with the 'displayField' and 'valueField' configs.</p>
  
  <p>Resetting the store via <code>cmb.bindStore(newStore)</code> works great.</p>
  
  <p>Setting <code>cmb.displayField = 'newfieldname';</code> also works great.</p>
  
  <p>However, <code>cmb.valueField = 'newValField';</code> does not work.  The combo displays the right stuff, but when i select an item, the value is using the old valueField value, not the new one.</p>
  
  <p>I've tried:</p>
  
  <ul>
  <li>doing a <code>cmb.reset()</code> afterwards</li>
  <li><code>Ext.apply(...)</code> </li>
  </ul>
  
  <p>Is it because <code>valueField</code> is somehow special because it is a required field?  Is there some special way to set a config value on an Ext-JS component I don't know about or is it just not possible to change the value of 'valueField'?</p>
  
  <p>FYI - Here is my code:</p>
  
  <pre><code>    comp.bindStore(Ext.create('Ext.data.Store', {
          fields : [ {
              name : 'abbr',
              type : 'string'
          }, {
              name : 'name',
              type : 'string'
          }, {
              name : 'slogan',
              type : 'string'
          } ],
          data : [ {
              "abbr" : "AL",
              "name" : "Alabama",
              "slogan" : "The Heart of Dixie"
          }, {
              "abbr" : "AK",
              "name" : "Alaska",
              "slogan" : "The Land of the Midnight Sun"
          }, {
              "abbr" : "AZ",
              "name" : "Arizona",
              "slogan" : "The Grand Canyon State"
          }, {
              "abbr" : "AR",
              "name" : "Arkansas",
              "slogan" : "The Natural State"
          }, ]
      }));
  
      comp.displayField = 'abbr';    // THIS WORKS
      comp.valueField = 'abbr';      // THIS DOESNT WORK
  </code></pre>