title: Only save changes in PHPExcel
body: <p>I am using PHPExcel for editing a large XLSX file (~230kb, 20 sheets). They way I do it yet is:</p>
  
  <ol>
  <li>load the file (create the PHPExcel object)</li>
  <li>do the changes on the PHPExcel object</li>
  <li>overwrite the XLSX file with a new one</li>
  </ol>
  
  <p>This is very slow (~6 seconds), although there are only small changes (some cells in just one worksheet).</p>
  
  <p>Is there a way to save just the <strong>changes</strong> in the file? Or if not, with another library?</p>