The Mercedes manual suggests you change your transmission fluid every 30 K miles - more frequently with hard use (i.e. city driving, very hot climate, pulling a trailer). The major danger in changing the fluid is getting lint or dust into the transmission, so be compulsively neat when you do the job. Unless you put the car on sturdy blocks or jackstands, you will have difficulty getting to the pan and to the torque converter to drain the fluid. Use a small funnel to pour the fluid in through the dipstick hole and protect it from bits of the sound insulation on the hood that might fall in. You also need to be able to turn the engine over by hand to get the torque converter drain bolt to line up with the hole in the casing - make sure you can do this before you start draining the pan. You're right, it is pretty easy to do.