Originally Posted by insightbrewery
Drain and refill. Any latent problems that become noticeable were there already and just because it shifts smooth with old fluid, doesn't mean it wasn't going to die soon.
Change filter, pan gasket, and drain and refill with fresh fluid. Doing these things with some regularity is how you keep transmissions going.
Drain and refill.
If you're really concerned, do it again after a month or so.
If you're still worried, go ahead and do a complete exchange using the cooler supply/return lines.
Some people (somewhat mistakenly) refer to the complete exchange as a "flush"
Yes, it is a "flush" in that it replaces all fluid in the system, but it is not a "power flush" where an external machine forces the fluid through.
Done properly, the engine is running, and as the old fluid is pumped out of the cooler outlet (by the transmission's pump), the machine replaces that fluid into the cooler/pan return line.
The cooler return line runs directly to the pan... same as an old transmission with a dipstick.
After fresh fluid begins being pumped from the cooler outlet, the system is closed up, and run to the proper temperature to check the fluid level.
This needs to be done whether it's a full exchange or a simple drain and refill.
Personally, at almost 200k with no maintenance, I'd opt for the complete exchange, then continue with a drain and refill every 50k.