Hello Folks:
I am a very competent DIYer. I'm one of the old guys and have been turning wrenches for over 50 years. Of course, one must realize their own limits and not accidentally do something that will do harm. That said, changing the fluid on an automatic transmission is not very difficult.
I say this not as some kind of weird joke or play on words, but all you need to possess is that "you know what you're doing".
It is my belief that anyone with a better than average understanding of mechanical things can change the fluid in a modern German car. I did it multiple times in my wife's 2006 C230 and never found it difficult (and I even used to drain the torque converter). That MB transmission required a special adapter that fed oil into the bottom of the pan. Fluid level is determined by a "stand pipe" inside the transmission pan. Once the fluid level rises above the stand pipe it overflows out of the adapter that you have inserted.
Just yesterday I changed the fluid on my wife's 2011 BMW 328i (my reason for replying to this thread). After just 22,600 miles the fluid was DARK BROWN! Here I thought I was being my typical OCD self and was wasting my time and money. But, based on color at least, the fluid absolutely needed to be changed. I not only drained the fluid, I also pulled the pan so I could replace the filter. The BMW was easier to refill than the MB because it's filled via a plug on the rear of the transmission. It doesn't require a special adapter like the MB did.
There are some issues/procedures you must follow (on both the BMW and MB).
1) The transmission must be at the proper operational temperature. Refer to the Bentley manual for your model and transmission type. My wife's car has the GM 6L45R transmission. Depending on the model some may have a ZF transmission. One of those handheld trigger pull infrared sensors is ideal. No need to hook up to the OBD port.
2) Before starting the engine, refill the transmission until fluid overflows out of the fill hole. At this point, start the engine and shift the car in and out of reverse and drive several times, then put it back in park. Leave the engine running and refill until fluid overflows out of the fill hole. With engine still running, re-insert the fill plug and tighten.
3) The car must be "level". Here's where it becomes a bit ambiguous. What is "level"? The car itself, or the transmission? As you know, many engine/transmission assemblies are not level (length-wise); the plane through the center of the engine/transmission is higher at the front than it is at the back. Based on measurements using a small level on the base of the transmission case where the pan bolts up to, I estimate the transmissions canted upwards in the front by about 10 degrees. I raised the rear of the car in an attempt to get the transmission case level but it required me to raise the rear of the car much higher than the front. Much higher. I pondered this for awhile and came to the conclusion that NO ONE, not a BMW service center or the best Indy mechanic is going to raise the rear of the vehicle such that the transmission case is level. This service will be done with the car up on a drive on lift, or a side access body lift. Both will result in the car (not the transmission) being the thing that's level. It is my belief that this is what determines "level"; the car, not the transmission.
In summary, my wife's 23K mile old factory fill ATF was in surprisingly bad shape given its color. I'm really, really glad I changed it.
Also, if you enjoy working on cars and have a deserved reputation that you "know how things work and you know what you're doing", changing the ATF on a BMW or Mercedes is not that difficult. Refer to a manual for specifics, don't rush, and think about what you're doing (e.g. what determines "level").
Unless you are one of the fortunate ones with a lift, the worst thing about changing the ATF is that the job is always messier than you think it will be. You're going to end up spilling a 1/4 quart of oil on the garage floor no matter how careful you are. Just 8 ounces of oil makes a surprisingly big mess.....and you always seem to end up laying in it!
For the MB you'll need this. For the BMW or any other car not requiring a special adapter this makes the job much easier.
Assenmacher Specialty Tools ATF1100-5 Drive Line Filler Set - 9 Piece
Best,
Scott