Ive changed my fluid twice since I owned this car. My car has 229k on the original trans. You're not going to get to my mileage with the same trans without changing it. Even if you only remove 30 to 40% its better than nothing and also there is still a filter in that should be changed.
I change it between every 50 to 60k.

I don't recommend trans flushes though. Ive seen that mess up transmission numerous times on various cars.