Most of them can have the oil changed.

My Bmw 525tds for example,1994 and a sealed box that required a good 20 mile drive to get it hot and then up on ramps and an assistant inside the car with the engine running and hand/ foot brake firmly on.

You then remove a fill plug on the side of the transmission and while the assistant works the shifter through the gate add oil until it dribbles out of the hole and jobs done.Another way is to connect hose to the trans cooler and let it pump out a specific amount whilst taking new in,obviously there is a trans filter to consider along with both approaches.

Downside is the oil costs a lot over say £150 here in the Uk,no scrimping and using the wrong grade as the box will fail or start to slip shortly afterwards.

Also at high miles there is a lot of clutch fibers floating in the oil and removing the oil takes these out of the box,the consensus is that replacing the oil can or will make the box slip at a high mileage.

It is 50/50 whether it slips or drives ok,the fact that these boxes and the one in my 94 Bmw monitor for slippage with speed sensors and once detected go into limp mode does not help.

Common thinking in Uk and Europe based forums is if approaching 150,000 miles or just under and no documentation to show it has been changed then leave it well alone.