Table mounted through input select

Good afternoon, my problem here is that I mount a table through an array received by ajax, I have 2 problems:

1 Every time I select an item in the select the table should reset and pick up the new data, the way I did it it is not adding without deleting

2º I have a text field where a number will be typed and has to be multiplied at all with the qtd_planned id. It will be of great help to be lost in this.

<label for="qtd_planejada" class="col-md-2 control-label">Quantidade planejada:</label>
  <div class="col-md-2">
  <form:input path='qtd_planejada' type='text' class="form-control"/>
   </div>
   <form:errors path='qtd_planejada'/>

  <table id="estruturaTabela" class="table table-striped table-bordered table-hover">
  <thead>
     <tr>
         <th> Item </th>
         <th> Descrição </th>
         <th> Quantidade Base </th>
         <th> Quantidade Planejada </th>
         <th> Disponivel </th>
         <th> Nome da UM </th>
         <th> Depósito </th>
         </tr>
   </thead>
      <tbody id="body">                 
      </tbody>
  </table>

<script>
$('select[id=produto]').change(function () {
    var itemCode = $(this).val();
    var tbody;  
    var tr ;
     var tr = document.createElement('TR');
         $.ajax({       
                type:"GET",            
                url:'/producao/estruturaProduto/' + itemCode,   
                success: function(retorno){
                  console.log(retorno);
                        if (retorno.length > 0) {                               
                            //TABLE ROWS
                            tbody = $('#body');
                            for (var i = 0; i < retorno.length ; i++) {
                                tr = $('<tr/>').appendTo(tbody);                                    
                                    tr.append('<td id="itemcode">' + retorno[i].itemcode + '</td>');
                                    tr.append('<td id="itemname">' + retorno[i].itemname + '</td>');
                                    tr.append('<td id="qtd_base">' + retorno[i].qtd_base + '</td>');
                                    tr.append('<td id="planejada">' + retorno[i].qtd_base + '</td>');
                                    tr.append('<td id="disponivel">' + retorno[i].disponivel + '</td>');
                                    tr.append('<td id="nomeUM">' + retorno[i].nomeum + '</td>');
                                    tr.append('<td id="deposito">' + retorno[i].deposito + '</td>');
                            }      
                        }                       
                }
             })

});