I have this code in a program that uses CVS HEAD gstreamer (from an hour ago):

  gst_element_set_state( gst->pipeline, GST_STATE_NULL );
                                                                          
       gst_bin_remove( GST_BIN( gst->pipeline ), gst->decoder );
  gst->decoder = gst_element_factory_make( "spider", "decoder" );
  gst_bin_add( GST_BIN( gst->pipeline ), gst->decoder );
  gst_element_link_many( gst->src, gst->decoder, gst->sink, NULL );
                                                                          
       g_object_set( G_OBJECT( gst->src ), "location", uri, NULL );
  gst_element_set_state( gst->pipeline, GST_STATE_PLAYING );

The idea is to reuse the src and sink elements of the pipeline to play
another file.  The decoder (spider) element has to be created fresh in the
case that the file format has changed (otherwise it doesn't work).

What's odd, though, is that when the pipeline goes back to the PLAYING
state, it picks up where it left off.

That is: if you 'switch' to the same song, there is a slight pause and no
other effect.  If you switch to a different song, then the song starts
playing at the timepoint that you *stopped* playing the song before it.