What mileage should transmission fluid be changed?

I probably would have changed at 30,000 or 60,000. If you have the owner's manual look in it for the recommended service interval.

Do it now, drop the pan and change filter and clean off pan and magnets and new gasket. Refill and then about 1000 miles later drain and refill again I would not flush, but drop and refill a few times until you have clean fluid in there. I have done this many times with customers who have never done maintenance at the correct intervals. This has always worked and my customers understand the return trips.

At 158,000 miles it is long overdue.