Do it. 100,000 miles is 50,000 too many.
You'll hear both sides of this argument, but generally members here will fall on the side of "do it". Any perceived negative impacts are most likely the manifestation of issues that are already present and simply coincidental to the fluid change. MY opinion: full fluid change + transmission filter change.
Here is a video on a simple method for doing the fluid change yourself:
. It can be accomplished in your driveway. If you have the resources to get the car off the ground (jackstands, ramps, etc...) also do a filter change. Most people on here will also recommend using Dexron VI fluid. The general consensus is that DexVI is far superior to the DexIV your vehicle shipped with. I'm switching to MaxLife tranny fluid that says it's DexVI rated. Less expensive than what you can get from the dealership. Walmart's price is in the $18 range. You will need 4 gallons if you do the fluid exchange yourself. DO NOT get a transmission "flush" at a service center or dealership. Most of those places use a machine that backflows the fluid which can have some very serious negative effects. There are multiple write-ups on this site for doing the fluid exchange as well as numerous videos on YouTube. @MAY03LT videos on YouTube are usually on point and he'll point out his own mistakes so you don't make them.
My friend has an 04 envoy, with 300k miles and only serviced his trans at 200k miles. let me tell you, don't be like that guy. At the very least do what @HARDTRAILZ said, fluid and filter cant go wrong
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.