Forget having a shop use a hose down the fill tube to extract the old fluid. For our automatics, doing a drain followed by flushing the upper trans cooler line at the rad uses the transmission to pump fluid through the entire trans, changing gears a few times to open the channels. It's in the FAQ here. It's not at all a messy job and just requires a quick visit under the car to do an initial drain at the plug. You don't have to worry about getting every drop out. It uses 2-4 times the fluid depending on how dirty things are. If it's really bad, do it twice with some good driving in between. The brass flange on the plastic rad is sturdier than you might think and, along with a little penetrating oil, can be opened using a counterhold wrench (squeezed in your hand against the other wrench) with little fear of cracking the tank. Clear vinyl tubing (easily found in hardware/plumbing stores and sometimes available by the foot) and a few marked milk jugs is all you need. Once you do it the first time and realize it's almost as easy as an oil change, you'll want to do it more often.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now