If your ports have already been assigned, they must be unassigned before choosing them again.
detail: From the ports tab – select the port and choose unspecified for each port on both client and server – now you can re-designate (assign) a port to a new line. Each port once assigned cannot be chosen again unless it not assigned anywhere else in the test. Often times rewiring a test harness can require that the actual configuration in the GUI gets modified – this may require a move of the assigned port.