Yes, it is possible to write a new IP address of connected devices via scripting during runtime. This feature has been implemented in Vijeo Designer V5.0 SP1.

Below sample script will set the IP addresses for any driver, not just the Modbus driver.

// --------------- Write some data ------------------------------------

// Length is given by NewIP_InputData.length

int [ ] NewIP_InputData = {10, 187, 225, 135};

ModbusEquipment01.openConfig();

ModbusEquipment01.setProperty( "Con_IPAddress", NewIP_InputData );

ModbusEquipment01.commitConfig();