Get her done!! I did mine at 164,000km give or take. Did the filter as well and will likely never do it again as I used much better quality fluid than the factory. Use the fluid exchange method highlighted on this or the old site.
I did a similar fluid exchange on my 1993 K1500 with the 4L60E. 374,000km, appears untouched. Fluid was very dark, smelled disgusting (as far as old ATF goes), did not smell burnt though and the transmission was shifting quite poorly. I could not accelerate to pass anyone on the highway as the engine revved and the trans just slipped. I pumped out the old and added generic conventional Dex 3/Merc fluid. I did not change the filter as I wanted to see how it shifted first. After several miles, the thing is back to shifting as great as any old transmission should. Yes it has a 3-4 flair but the truck is not worth adding anything more.
Point is: If the transmission is failing, new fluid cannot stop that and neither can leaving the old fluid in. If the fluid is failing and you catch it in time, the transmission can keep on ticking.
Even if you skip the filter, fluid is cheap and it is so easy to pump out the old and pour in some new stuff.