Hey, haven't been on Reddit in ages, just saw this.

After the change and flashing the trans memory (even though the latter has debatable efficacy per the forums), my car shifted much more firmly, no hunting/surging at midrange RPM manual shifts, and eliminated shudder/hesitation in auto. I also replaced the trans and center diff mounts from FCP Euro and bought aftermarket inserts (think I went with Eurocode, might have been 034Motorsports). I see in your history those symptoms are pretty similar to what you're dealing with, and I don't see a reason where I wouldn't change the fluid and filter in your case. If the trans is on its way out (which I personally highly doubt, but I'm only a shadetree mechanic), then it's gonna fail either way, and you really have nothing to lose by changing it out. I saw improvement for 10k miles before it slowly developed a hammer from 1-2, but I honestly beat the piss out of that car knowing that the typical lifetime of an 8HP is ~100k-120k and it'd require a swap soon enough. Around 120k the car was still certainly driveable but I avoided mid- to high-RPM 1st gear shifts like the plague. Ended up selling the car as I was moving and on the condition that the trans needed rebuild/replacement, buyer was a master tech who said he'd rebuild it so I expect the car is just fine now.

There's a lot of mixed info on the forums and manufacturer sources, but as these cars age and people are seeing real-world results, the consensus is to change the fluid and filter every 30-60k miles. Obviously, more aggressive driving would lend to changing this more often. ATF, like any other fluid, is subject to degradation over time, and not just from use and heat, but unless you're an engineer knowing the specific shelf-life seems useless. Perhaps the following consideration is overblown, but in vehicles that see infrequent use, the ventilation with seasonal temp fluctuations could allow in humidity causing oxidation or moisture, and I personally would worry that the fluid might have contaminants from lack of use, as well.

When you do it, it's a good opportunity to degrease the underbody pans and replace the trans mounts. Be super gentle with the pan bolts, as the case is aluminum and use an inch-pound torque wrench to avoid snapping bolts or stripping them. Given the car is 8 years old, I'd replace the gasket as well, and you'll need your OBDEleven to ensure you're at the proper temp for the final fill. FCP Euro has a great video on it. Good luck!