Dealership said it could negatively affect the transmission. Private mechanic said the same thing. The transmission will last longer with fewer issues if I didn’t change the fluid. Unless you change it from the day you bought it, it’s not recommended.
I'd disagree on that one. I'd avoid a "power flush" though. New fluid will help as will getting rid of all of that clutch material in the filter, on the bottom of the pan, and sticking to that magnet too.
I had a 1995 Jeep Cherokee as a teen, bought it when it had 171,000 miles and was 10 years old back in September 2005. The following summer I drained the pan and refilled it about 5 times. It had a drain plug, and unfortunately I never changed the filter or cleaned the pan as I was still learning and not as savvy as I am now. But I did so many pan drains that I turned dark brown fluid into bright red fluid throughout the entire system, and it performed better than ever! Not one problem encountered doing so and I drove it until I sold it in September 2011.
I did a pan drop and filter replacement on my Yukon last summer, first time at 40,000 miles. No one will convince me that NEW fluid will cause more problems than OLD fluid!