It's bad sometimes kause its the only thing holding it together kause people don't have it changed often enough. I always change my fluid every 10-15k no problems, I've never had an e4od last less then 185k but other than having a huge trailer and an under powered truck. It was an f150. If proper maintenance is done you will be fine, I flushed my current truck kause it was done til PO got sick, it sat for 2 years is why I had it done.
If a transmission is on it's last leg, changing the fluid or flushing will usually kill it, since the new fluid won't have remains of suspended friction material. A transmission in that sad of state would have needed to be replaced or rebuilt anyway.
Regular maintenance on a good transmission won't hurt it. I had my 4R100 flushed/filter change last year without incident, and the truck is daily driven with a 8,000 lb curb weight.
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