The concern with Dex/Merc fluid is that there is no regulations on the fluid any longer, they were up when licensing of the product ended almost 10 years ago. Now the manufacturers can do anything they want and say it'* usable for those applications.
Dexron VI is still licensed by GM. Also, the only real 'shock' is having clutch material that is suspended in the fluid pulled out, and new detergents being dumped in. I prefer to pump out as much as I can if I am exchanging fluid through the fill tube (4-5 quarts can be pulled usually) and refill. If that causes the tranny to go, then it was on borrowed time anyway. No difference whether you get it all at once or a quart at a time.