What code will force a reselection in TinyMCE 4.6?

I'm having a problem with TinyMCE 4.6. I've implemented a custom button that bumps the font size of selected text:

ed.addButton('finc', {
           image: '/tinymce/plugins/zackel/button_images/big.png',
           title: '+ font size',
           id : 'finc',
           onclick:function(editor,url) {
             console.log("************ In finc: ", ed);

             var delta;
             var currentFontSize = new Number($(ed.selection.getNode()).css('font-size').replace('px',''));
             console.log("************ finc: currentFontSize = " + currentFontSize);


             var node = ed.selection.getNode();  // <======= LINE 565
             var nodeName = node.nodeName;  // for example 'DIV ' or 'P'
              console.log("************ finc: node is ", node, "nodeName = " + nodeName);

             if (currentFontSize >= 24) {
                 delta = 2;
             }
             else {
                 delta = 1;
             }

             currentFontSize =  currentFontSize + delta;
              console.log("************ finc: New font size = " + currentFontSize);
             ed.formatter.register('incfont', {
                 inline : 'span',
                 styles : {'font-size' : currentFontSize + 'px'}
              });
              ed.formatter.apply('incfont');
              console.log("********** finc: posting to val box  " +  currentFontSize);
              $("div#px_val button").text(currentFontSize + 'px');  // show value in value box
           }
        });

If the text is initially in a P the button works fine but puts the text into a span inside the P when it's done. If I then just hit the button again it fails because the node it brings back on line 565 is still the P, which still has the original font size. So if he initial font size is 16, it goes to 17 but then every bump after that stays at 17. If I deselect the text after bumping it and reselect it, line 565 gets the span and the bumps work every time.

How can I force a reselection from my code, so 565 finds the span the second time instead of the P, without me deselecting and reselecting the text?

Thanks