In the Mercedes WIS, if the TC has a drain plug, drain it and add that amount back in when you fill the transmission. If the TC doesn't, forget it.
You might remember that Mercedes once recommend a life time fill, and there are older cars running around with the original fluid. Mercedes changed it to 40K and 40K thereafter, but the reality is most people will only do one.
Flushing isn't a good idea, because you are forcing the fluid out often through tiny orifices which might trap some small particles. I doubt Mercedes recommends it. The main reason to even bother draining and putting in new fluid is to change the filter in the pan. The inside of a transmission is a clean environment, and assuming there aren't any operational issues, the transmission fluid should be good forever.