In order to move a RadTreeViewItem from one parent to another you will need first to remove it from the visual tree. Here is an example how to move an item in a scenario with statically declared RadTreeViewItems.
RadTreeViewItem itemToMove = GetItemToMove();
RadTreeViewItem oldParent = itemToMove.ParentItem;
RadTreeViewItem newParent = GetNewParent();
Please give this approach a try and let me know if it works for you.
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