I have been told, if your transmission has not received regular service through its lifetime, a flush could possibly loosen deposits that are better off left alone. I would agree with that. I have a link for a Do it yourself procedure that will get 99% of the old fluid out. Don't shoot me its for a Ford, but i use this for my other truck. The steps for draining should be similar, but I would use caution, and make sure you know what type and how much fluid you need for your application.
http://www.ford-trucks.com/article/i...nsmission.html
Dusty