I am convinced, and colleagues that are auto box service/repairmen, always recommend a change at no more than 40K km for any type of auto box, regardless of what the manufacturer says. Fresh fluid can't hurt, especially considering the thermal stresses is subjected to. No-one recommends flush, but partial changes, or, if you are keen and diligent, you can get to a full change by opening the circuit in one of the cooler outlets and therefore replacing more fluid than with the standard drain/refill (including the filter/strainer).
The Audis that we service, that mount the 6-speed ZF 6HP26A ("A" is the Audi derivative, with the transfer/front diff in the same case), fall into that category as well. For those we drain and refill only as above, no flushing.
I would never flush on an unknown box, danger of freeing impurities that can complicate things is high.
If you do open the box and proceed to a full cleanup then is ok.
Now I got a question for the fellas ... what ATF you use in yours? I am not interested in brands, but type. I've replaced mine (partial drain/fill + filter/strainer, this was before I've perfected the method above) when I bought the car (239K km) and then I've done another one at 254K km. I plan the next in another 30K km, if the box holds as much ....
I've used Mobil 320 Dextron III type on both occasions, but now I have learned there are many "varieties" of Dextron III, ranging from C, D, H, J, and so on. Anyone has a clue that is the correct type for our old gearboxes?
I got a friend that made a total flush and went with Dextron VI (the full-synth) and he says his box changes much better, but I am afraid to try ....