My mechanic did this on my old VS. What he did was disconnect the cooler line, aimed it down into a drain pan, then right next to it he'd connect the tran oil filler drum to refill and start the engine. As the engine is running, oil is coming out of the cooler pipe and at the same time he's pumping new oil into the auto while carefully monitoring the colour of the oil coming out of the cooler hose, once the colour is nice and new the entire system has been drained out. I can't remember exactly how many litres it was but it cost me about $140 for the service I think.
It's something that I will be doing on my VX as well because the difference it made was increadible, it remarkably felt like a brand new auto, the downshifting was increadibly responsive and firm and upshifting really gentle unless I pushed it. Definately a great thing to do to your tranny IMO.