The tag of a tree item is meant to be a unique ID of the item, and just like you cannot change the ID of a control or panel after it is created, you cannot change the tag of a tree item once it is created.
Unlike the ID of a control or panel, you can influence the value of the tag (ID) of the tree item through the TagBase parameter of InsertTreeItem().
If you just want to associate a string (that does not have to be unique with respect to the other tree items) you should set the ATTR_DATA_TYPE of the tree to VAL_STRING. Then you can set and change the string value of the tree item with SetTreeItemAttribute() with ATTR_CTRL_VAL.
