Changing your fluid cannot cause any damage by itself, if everything is fine inside the transmission… The issues start to appear when you change the fluid if your clutches are already worn or damaged from use. If you go and replace the fluid after they are already worn out. this can cause slipping…
Long answer:
Short answer:
First, there is really no such thing as a truly “sealed” transmission… What most people call sealed transmissions are simply dipstickless transmissions… Likewise, there is no such thing as “lifetime fluid”… There is only fluid that is expected to last for the anticipated service life of the transmission, which manufacturers generally consider to be around 150,000 miles… Many transmissions can and do last much longer than that…
Really really really long detailed answer:
This was on a Toyota thread, but the basics works here also…