As far as mercedes benz are concerned, the transmission fluid is for life and doesn't require any servicing. This basically applies to all transmissions that don't have a dipstick.

Nearly any car you buy now has this same transmission policy, advancements in transmission design and fluid technology have increased their longevity.

So as far as the manufacturers are concerned, if it's not faulty, leaking fluid or have any abnormalities, then you simply don't touch it.

As long as it performs beyond it's warranty period.

Getting back to your car, there's no way I would personally just flush and change the fluid.
The filter is in there for a reason, makes no sense at all why someone would renew the fluid without replacing the filter, you wouldn't change the engine oil without replacing the filter?

I would suggest having the pan dropped and the filter replaced, should take less than an hour to do and about 3 litres of fluid.

ozzy