This is the 6L80/90, right?
I think 50k on that unit is recommended? Maybe as little as 25k if you haven't disabled the AFM/DoD and do heavy towing or a lot of city driving?...see the 6L80e thread from
@NickTransmissions for more info.
On the old 4Lxx and earlier models there's this idea that changing your transmission fluid at high mileage will cause it to start slipping because the "old ATF has 'grit' to help the clutches engage." I've heard even reputable mechanics mentioned this like it's the Gospel. I certainly believed this myself. As far as I know that is false, and is an old-wives tale. This should be like a PSA.
There's absolutely no way a friction particle that got abraded away and is floating around in the lubrication circuit can do this. The friction material is bonded to the backing substrate. Keyword: bonded. After it falls off it doesn't do anything. Not to mention all the grit would cause damage to bushings, bearings, valve body valves. Yuk. The proof is converter failure of a 6L80e causing extreme scoring and damage to the pump body. :-(