Setting csrf_regenerate as TRUE

Thanks Martin for your help. I am posting the solution in case someone needs it:

PHP Code:

$('#department_id').change(function() {

    var department_id=$("#department_id").val();
    var domain=$("#domain").val(); 
            
    $.ajax({
        'async': true,
        'type': "POST",
                url: domain + "index.php/project/get_scheme",
                data: {department_id: department_id, csrf_test_name: csfrData.csrf_test_name},
                dataType: 'json',
                success: 
                function(j){
            
                  setCSRF(j.csrfTokenName, j.csrfTokenHash);

                  if(j.schemes.length >= 2) {
                    var options = '';
                    for (var i = 0; i <= (j.schemes.length-1); i++) {
                            options += '<option value="' + j.schemes[i].id + '">' + j.schemes[i].name + '</option>';
                    }
                    $("#scheme_id").html(options);
                    $('#scheme_id option:first').prop('selected', 'selected');
                  }
                  else if (j.schemes.length == 1){
             
                   options += '<option value="' + j.schemes[0].id + '">' + '------------------------------------' + '</option>';
                    $("#scheme_id").html(options);
                    $('#scheme_id option:first').prop('selected', 'selected');
                  }
                  else{
                    $("#scheme_id").val(j.schemes[1].id);alert(j.schemes.length);
                  }              
            }
          });
  
   }); 


Like this, other drop-down boxes are filled up through ajax call. 

Now when I try to submit the form, again "Action you have requested is not allowed" message comes up. The hidden csrf field generated by form_open doesnt get updated with the latest csrf token value.