1999 Toyota Corolla Transmission fluid, to change or not to

Hi guys, wanted to ask your opinion on 2 things related to this car (my sister-in-law's Corolla). I only recently started helping her out with some of the servicing. She's had the car since it was brand new, and it has about 138K on it now. I have no idea what it has had done to it in the past, and so far, she hasn't been able to find the maintenance records (she's moved a couple of times). She did tell me that most of the servicing has been done at oil change shops. She can't recall if she's had a transmission flush or fluid change done.

First, I've heard that in general, it's better to just drain and refill than do a full flush, since flushing can stir up particles and potentially cause damage. It's also cheaper to drain and refill.

Second, I recently read that if the car is high-mileage and has NOT had the transmission fluid changed out, and the transmission is not having problems, it may be best to leave it alone. This was due to particles in the fluid providing some of the necessary friction for the transmission clutches and other parts to function properly, and changing the fluid out would remove those particles and then you might see slippage or other problems. Since we don't know the history, I'm kind of afraid to do anything. I have a feeling that it hasn't had a drain/refill for a long time, if at all, yet it's still working, and I'm afraid to do it and cause it to suddenly have problems. On the other hand, I'd hate for her to have a sudden transmission breakdown due to lack of maintenance. What would you do? Is there a way to perhaps take a "sampling" of the fluid and see how it is? A couple of weeks ago, I had her tires rotated and oil changed, and the tire shop that did that said her transmission fluid should be replaced, but of course, they wanted to do the full-on flush and charge me $125, so I said No Thanks. But they didn't say anything specific about the fluid's condition, other than that it should be flushed.

Thanks for any help!