I have had people give me anecdotal stories (friend of a friend type thing) about changing the transmission fluid in vehicles with 150,000+ miles, especially those that haven't had regular transmission maintenance. The usual story is something along the lines of "older American make care with 200,000 miles, a week after we changed the fluid, the transmission went out."
Is there any truth to this? Best answers I could find searching the internet were pretty inconclusive at best, such as yahoo answers with unhelpful answers like "if it ain't broke, don't fix it."
With my vehicle in particular (2000 Chevrolet Venture), I had a transmission repair and fluid change at 130,000 miles, and am now at 190,000 miles (so it is a little overdue for transmission maintenance). I've had a little hesitation in shifting: I talked to my mechanic and he likely said it's a sticky solenoid, and he suggested a cleaning transmission treatment (such as Seafoam), but he hasn't looked at it. If I understand correctly, such treatments are usually best followed by a fluid change.
If I have to do a major repair at this point, I'd pretty much declare the vehicle "junk" instead. So I'm fine with doing some minor maintenance to keep it going for a couple more years, but don't want to do anything that will possibly harm the transmission or make the problem worse.