This is an excellent article on a complete transmission flush including the filter that convinced me to do mine.
http://www.xoutpost.com/x5-e53-forum/...smission+flush
One can see all of the sediment in the bottom of the pan and on the magnets. If you only change fluid, there is a chance of dislodging this sediment and causing transmission failure. If you are going to change fluid I recommend you drop the pan and change the filter too.