That's fairly difficult to say. It could have been coincidence.
I've never seen sludge inside the gearbox... only in the sump but then I've not stripped down a gearbox that has done 200,000 miles without an oil change!
I personally would have no hesitation in changing the oil so long as it's done properly.
Get it wrong and you can do serious damage.
The O-ring design is horrible and I'm surprised that ZF have designed a gearbox that can fail so dramatically from one O-ring failure. Maybe they could have designed it better so that the O-ring was more supported, maybe they should have used better material for the rubber... I'm no engineer. I think if ZF were revisiting this now, they'd probably re-design it with a bigger O-ring and re-machine everything accordingly.
Oil changes are good. I would say that 60 or 80k for an auto gearbox oil and filter change is a good idea. I think maybe the same time as the timing belt would do but that's just my opinion.
Here's a diagram from ETOS with the parts that I need highlighted in red.
From Audi, you can't buy the drum separately. I'm hoping that you can from ZF (therefore Chester Transmissions). Finding the O-ring on the parts page is a nightmare as VW/Audi have just put all of the O-rings on one diagram... with no clue really as to where they go!
MG