<script type="text/javascript" charset="utf-8">  
  $(document).ready(function(){
 
    var newText = 'This is the sub-question header';
 
    $('#question{QID} table.subquestions-list thead td:first').addClass('sub-question-header').text(newText);
  });
</script>

td.sub-question-header {
  font-size: 11px;
  padding: 4px 6px;
  color: red;
  text-align: right;
}

<script type="text/javascript" charset="utf-8">  
  $(document).on('ready pjax:scriptcomplete',function(){
 
    var newText = 'This is the sub-question header';
 
    $('#question{QID} table.subquestion-list thead td:first').addClass('sub-question-header').text(newText);
  });
</script>

<script type="text/javascript" charset="utf-8">  
  $(document).on('ready pjax:scriptcomplete',function(){
 
    var newText = 'This is the sub-question header';
 
    $('#question{QID} table.subquestion-list tr.ls-heading td:first-child').addClass('sub-question-header').text(newText);
  });
</script>