title: Change (not init) a UIBarButtonItem identifier programmatically?
body: <p>In IB I can set the identifier of a UIBarButtonItem to 'play' which adds an image of a play button (right-pointing triangle).</p>
  
  <p>Is there a way to change this image programmatically?  I want to change it to 'pause' when the play button is pressed.</p>
  
  <p>I know you can initialize a UIBarButtonItem with an identifier but I've yet to find a way to change it after it's been initialized.  Is this even possible?</p>
  
  <p>The only thing I can think of is to remove the old button and initialize a new one in its place, but this hardly seems efficient.</p>
  
  <p>Any thoughts?</p>