For connected transformations, if you use port names in an expression, the Designer updates that expression when you change port names in the transformation. For example, you write a valid expression that determines the difference between two dates, Date_Promised and Date_Delivered. Later, if you change the Date_Promised port name to Due_Date, the Designer changes the Date_Promised port name to Due_Date in the expression.
You can propagate the name Due_Date to other non-reusable transformations that depend on this port in the mapping.