[whpalmer4]

You want to make a new diagram style. File->New Resource->New Diagram Style..., select the Lines diagram style. Make your changes. File->Save As... your new diagram style (it will default to the Diagram Styles directory so it shows up in the Diagram Style chooser). Now you should be able to apply that new diagram style to your existing document with Format->Choose Diagram Style...

See "Using diagram styles" in the built-in help function for more details.