I'll give you my advice, but first I want to share that I've been hearing all my life how changing the ATF in a high-mileage transmission that's never been serviced is a recipe for disaster. I've had techs tell me very earnestly how fresh, clean fluid will disturb all the dirt that's in there now, change the wear patterns on the clutches, and increase the line pressure to the point where the dried, cracked old seals blow out.
I suspect that there are three factors related to that advice. First and foremost is the fact that dirty, worn-out lubricants lead to failures, so some dishonest shops urge you not to change the fluid in the hopes that the transmission will fail sooner rather than later.
Second, many drivers tend to ignore their transmissions until they start to slip or exhibit other symptoms of failure. Then they rush to the mechanic, hoping that a "transmission tune up" will fix it right up. A good mechanic will usually explain at this point that a fluid change is a waste of time and money, and repairs or a rebuild are necessary. There are customers who will "push" the mechanic to change the fluid anyway, hoping for a miracle. When it doesn't help, the customer can be resentful and unhappy, and they may forget that the mechanic advised against it in the first place. The customer may even claim, "It was fine until you changed the fluid". So good shops can be a little wary about changing the fluid on a long-neglected transmission.
Finally, there's the whole "flush" issue. Many shops buy expensive "power flush" machines based on the idea that they can use them to quickly and easily perform transmission flushes, for which they can charge customers a lot of money. So they tend to push transmission flushes, both to pay for the machine and to boost profits. The problem is that I consider a power flush to be a risky procedure. Forcing fluid under pressure through the transmission may be quick and easy, but I think it's also likely to disturb static dirt deposits and cause them to begin circulating, possibly leading to premature failure. Again, a good mechanic will almost always caution you against this kind of procedure.
I personally feel that the key is whether or not the transmission is in good condition. If it shifts as it should and there's no reason to suspect that failure is imminent, I always change the ATF fluid and filter when I buy a used car, regardless of how many miles are on it. If the fluid is still bright red and doesn't smell burned, you can just change what's in the pan, and as long as you do that on a regular basis...say, every two to three years, depending on mileage...you should be fine.
On the other hand, if the fluid is a very dark color and/or smells burned, I like to get all of it out of there. If the car has a drain plug in the torque converter, you can easily do a complete (or nearly complete) fluid change very easily. If there's no drain plug, I'll perform a "passive flush". I'll disconnect the transmission cooling lines, connect hoses to them, put the intake hose in a bucket of clean ATF, and the output hose in an empty bucket. Then I'll start the engine and use the transmission's own pump to do the job. When the oil starts to come out clean, you re-connect the hoses and verify that your fluid level is correct.
Your car probably doesn't have a dipstick. You can buy one from that auction website, or that other big website named after a river in South America, but after a fluid change there's a specific procedure you have to follow which includes warming the transmission to a specific temperature range before checking the fluid level. But the dipstick makes it easier to keep an eye on the fluid down the road, once you know where it should be.
