I just do drain/fills every 30K miles.

When it comes to high mileage autos that have never been serviced...I would just leave them alone. So much friction material suspended in the fluid at that point that a flush can sometimes cause more issues than just letting it be. I have personally witnessed two cars have issues after a high mileage flush after at least 100K of non service to the transmission.

At least a drain fill is ~4QT, so you get some fresh fluid in the system but don't get rid of all the suspended friction material.

Just my opinion and my maintainence schedule with my own car.