Dynamo tries to update elements it has placed whenever possible rather than create new ones. This is almost always the case for most users, but you are right, sometimes you want to reuse an old graph and break the existing associations. (Issue tracked internally as MAGN-1581.)
Think of the association between the Dynamo graph and the Revit elements as being contained in the node that places the elements. In the case of the truss, it’s probably something like Beam.ByCurve. Your quickest option to reuse your graph by simply replacing the Beam.ByCurve node with a new Beam.ByCurve node when you want to “break” Dynamo’s association with what it made.