No, in most cases there is no need to flash the ECU when replacing an automatic transmission. Modern cars are equipped with self-learning ECUs that automatically adapt to the new automatic transmission. However, in some cases it may be necessary to update the ECU software or perform a calibration to ensure optimal interaction with the new automatic transmission. This depends on the specific car model and the type of automatic transmission being replaced. It is recommended that you consult your repair manual or consult a qualified transmission service technician to determine if ECU flashing is required for your specific application.