Changing the header of a CheckboxSelectionModel does not work properly. The sprite checker image will still show up with the new header over top of it. The issue stems from Ext's assumption that the css class of the header td can be named x-grid3-hd-{columnid}.
It leads one to believe that simply changing the header config will remove the checker image in the header. This is not the case because the id must be changed as well (to change the css class name).