I'm on the fence about this as well.
141k on the clock, bought her with 68k. Not sure if it was ever changed by the previous owner(s), but I know I've never changed it. Has had a large aftermarket tranny cooler on it since I purchased it. Since about 135k, I've reprogrammed the tranny slightly with HP Tuners...I didn't change any line pressures, but just increased some shift speeds and made some minor tweaks on how long it will hold a gear in the towing mode, as some of the factory settings have annoyed me a lot ever since owning it.
Tranny doesn't slip or have any issues whatsoever - and never has. Fluid is still pretty red. I've probably added 2-3 quarts to it since I owned it (about 1 when I did the cooler, another 2 from a leaky hose clamp I repaired).
I fear changing the fluid will just cause me issues. I tow short distances with it very often in the summer (around 5000lbs), longer distances once in a great while.
I just can't decide of which gamble to take - the leave it alone gamble or change the fluid gamble. I wouldn't do a flush, but rather pull the pan (clean it + new filter), then pull the output of the tranny cooler hose and fill new fluid in the dipstick tube as old fluid comes out the cooler method. I would love to NOT spend money on a new tranny this year.