Change dialog content after opening

Change dialog content after opening

My situation:
I have a long saving-proces. In my Ajax-call I open the dialog with the beforeSend method.

$.ajax({
     ....
    beforeSend: function () {
          $("#saving").dialog({
               bgiframe: true,
               modal: true,
                resizable: false                   
              }); }
...

This works fine.
In the dialog, I display "Saving..."
After the ajaxCall is succesfull I want to change the message and add a button "OK" to it.
  $("#saving").html("Data saved");
  $('#saving').dialog('option', 'buttons',
                  { "Ok": function() { 
                                 $(this).dialog("close");
                             }
                  });

But this won't work...
Is it possible to change the dialog-content on the fly or is there another jQuery component to create this behavior?