The brief
The E250 had reached the mileage where the automatic transmission fluid is due for a change, and the owner had brought it in proactively rather than waiting for the shifts to go off. Good call, the gearbox is one of the most expensive things on the car and the fluid is what protects it. The ATF lubricates the gears and clutches, cools the transmission, and carries the hydraulic pressure that does the shifting. Heat and friction degrade it over time, so it loses its ability to protect and to shift cleanly, which is why it's a scheduled item. Changing it at the recommended interval, along with the filter and the pan gasket that seal the bottom of the box, keeps the transmission running smoothly and prevents the overheating that wears clutches.
The diagnosis
A check of the fluid showed it darkening and tired, right where you'd expect at this mileage, with no metal or burnt smell, just used. The transmission itself was shifting fine, no faults stored, so this was a service rather than a repair. Confirming clean, even fluid wear like that is useful in itself: it tells you the transmission is healthy and the change is being done at the right time, before any harm is done.
The work
The transmission pan was dropped, the old fluid drained, and the old filter and gasket removed. The pan and the magnets were cleaned, a new genuine Mercedes-spec filter and pan gasket fitted, the pan torqued back up, and the box refilled with the correct ATF to the proper level following the fill procedure, with the fluid temperature checked so the level was set right. A road test followed to confirm the shifts stayed smooth and there was nothing leaking.
The outcome
Smooth, prompt shifts, fresh fluid at the correct level, and no leaks. The E250 went home good for the next transmission interval. ATF is a service item, and a transmission run on degraded fluid wears harder and shifts worse, so doing the full set on schedule is the cheap insurance against a far bigger repair down the line.