The first guy is useless but chrisfix is fairly accurate but he highly contradicts himself when he says that changed the fluid can't damage your transmission. He claims it's already damaged. That is true BUT the damage is that it's warm and has a lot of grit in there. It is damaged however it is still moving the car and not slipping. He even says this in the next sentence. That adding the new fluid could cause it to slip. So he personally doesn't consider causing a transmission to slip damaging it but most people would. The simple fact is that if you have a transmission with the dark black gritty fluid and the clutches aren't slipping at all , they might start slipping if you put fresh fluid in it. They have an even greater likelihood of slipping if you flush the transmission.