Dorman makes a replacement trans pan with a drain plug in it. If you want to change the fluid peacemeal, I would start by replacing the pan with the dorman one, along with a new filter, and then just do a drain and fill every oil change until you are satisfied with the color that is coming out. It will take more fluid to do it that way than just changing it all at once, but simply opening a drain plug and refilling the trans is quick and easy, and overall you will spend way less time doing that than trying to siphon the fluid up and out of the dipstick tube.