There is a theory that if you have over 80-100k miles on the tranny without flushing it then the fluid has a lot of friction material in it that is actually keeping the tranny from slipping. If you flush this out with fresh fluid you've lost all the friction material and there is a very good chance that your tranny will soon be slipping.
If you have relatively low mileage you should be fine. Otherwise just add a tranny cooler and enough fluid to fill up the tranny after the fluid has been pumped through the cooler.