Refresh Tree with Checkboxes

I have a tree (with checkboxes) that loads via XML. I select a new group and I would like the tree to return a new XML with the neccessary items checked. I can see that the XML is returning, but the tree is not reflecting the changes to the checkboxes. Any help would be greatly appreciated.

Steve

How do you reload the tree ?
Locally the following updates checkboxes’ states:

tree.deleteChildItems(0);
tree.loadXML(url);