The main purpose of oil in auto gearbox is to work as hydraulic fluid. If a flush change (renewal of whole oil content) is done properly there is very little chance you could do any harm. Personally I've removed the the oil line on oil heat exchanger and guiding that fluid to container while filling the pan with fresh fluid (drill pump) with engine running. (Of course normal drain and filter change done first.) When the colour of oil coming out of exchanger changes you know the whole content is new and turn off engine. Reconnect hoses and do a normal fill up with engine running and oil temp 30-50 C.