We can dynamically change the frozen property. See the code in index file

<?php 
require_once '../../../../php/demo/tabs3.php';
?>
<!DOCTYPE html>
<html>
  <head>
    <title>jqGrid PHP Demo</title>
    <!-- The jQuery library is a prerequisite for all jqSuite products -->
    <script type="text/ecmascript" src="../../../../js/jquery.min.js"></script> 
    <!-- We support more than 40 localizations -->
    <script type="text/ecmascript" src="../../../../js/trirand/i18n/grid.locale-en.js"></script>
    <!-- This is the Javascript file of jqGrid -->   
    <script type="text/ecmascript" src="../../../../js/trirand/jquery.jqGrid.min.js"></script>
    <!-- This is the localization file of the grid controlling messages, labels, etc.
    <!-- A link to a jQuery UI ThemeRoller theme, more than 22 built-in and many more custom -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css"> 
    <!-- The link to the CSS that the grid needs -->
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/trirand/ui.jqgrid-bootstrap.css" />
    <script>
        $.jgrid.defaults.width = 780;
        $.jgrid.defaults.responsive = true;
        $.jgrid.defaults.styleUI = 'Bootstrap';

    </script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>
    <meta charset="utf-8" />
     
    <script type="text/javascript">         
        jQuery(document).ready(function(){
            jQuery("#frozenchange").change(function(){
                if( $(this).val() == "2" ) {
                    $("#grid").jqGrid("destroyFrozenColumns")
                    .jqGrid("setColProp","OrderID",{frozen:false})
                    .jqGrid("setFrozenColumns")
                    .trigger("reloadGrid",[{current:true}]);
                } else if($(this).val() == "3") {
                    $("#grid").jqGrid("destroyFrozenColumns")
                    .jqGrid("setColProp","OrderID",{frozen:true})
                    .jqGrid("setFrozenColumns")
                    .trigger("reloadGrid",[{current:true}]);
                }
            });
        });
    </script>
  </head>
  <body>
          <div style="margin-left:20px">
      <div>
          <select id="frozenchange" class="form-control" style="width:220px;">
              <option value ="2">
                  Froze first two colums
              </option>
              <option value ="3">
                  Froze first three colums
              </option>
          </select>
      </div>
            <br/>
      <div>
          <?php include ("grid.php");?>
      </div>
        </div>
      <br/>
      <?php tabs(array("grid.php"));?>
   </body>
</html>

grid.php.
<?php
require_once '../../jq-config.php';
// include the jqGrid Class
require_once ABSPATH."php/PHPSuito/jqGrid.php";
// include the driver class
require_once ABSPATH."php/PHPSuito/DBdrivers/jqGridPdo.php";
// Connection to the server
$conn = new PDO(DB_DSN,DB_USER,DB_PASSWORD);
// Tell the db that we use utf-8
$conn->query("SET NAMES utf8");

// Create the jqGrid instance
$grid = new jqGridRender($conn);
// Write the SQL Query
$grid->SelectCommand = 'SELECT CustomerID, ShipName, OrderID, OrderDate, Freight, ShipAddress, ShipCity FROM orders';
// Set output format to json
$grid->dataType = 'json';
// Let the grid create the model
$grid->setColModel();
$grid->setPrimaryKeyId('OrderID');
// Set the url from where we obtain the data
$grid->setUrl('grid.php');
// Set some grid options
$grid->setGridOptions(array(
    "rowNum"=>10,
    "rowList"=>array(10,20,30),
    "sortname"=>"OrderID",
    "shrinkToFit"=>false,
    "width"=>700,
    "rownumbers"=>true
));
// set frozen property
$grid->setColProperty("CustomerID", array(
    "frozen"=>true,
    "width"=>100
    )
);
$grid->setColProperty("ShipName", array(
    "width"=>150,
    "frozen"=>true
    )
);
// Change some property of the field(s)
$grid->setColProperty("OrderDate", array(
    "formatter"=>"date",
    "formatoptions"=>array("srcformat"=>"Y-m-d H:i:s","newformat"=>"m/d/Y"),
    "search"=>false
    )
);
// Enable navigator
$grid->navigator = true;
// Enable excel export
$grid->setNavOptions('navigator', array("excel"=>false,"add"=>false,"edit"=>false,"del"=>false,"view"=>false));

// 
// Call the frozen cols method
$grid->callGridMethod('#grid', 'setFrozenColumns');
// Enjoy

$grid->renderGrid('#grid','#pager',true, null, null, true,true);