Ask any mechanic whether an automatic transmission fluid flush and replacement is necessary and you'll get two definitive answers: a qualified "yes" or an emphatic "NO". It's a controversial subject amongst mechanics because, well, there's a lot of disinformation out there.
To start with, the common argument against flushing is that it can "blow the seals" inside the transmission. This is true if you (wrongly) use a pressurized flushing system. Most shops that have experience with flushes do not use these outdated machines. Instead, they use newer machines that actually activate the pump within the transmission itself to have it flush the fluids for them.
The two most common machines are the Cooler Line and the Pump Inlet types. Both use the same basic method of having the transmission's fluid pump push the old fluid out. Both also use the vacuum created to draw new fluid from a reservoir and into the transmission as the old fluid is vacated. The difference between them is how they connect to the transmission itself.
Cooler Line Flush
The Cooler Line machines connect to the cooler lines on the transmission. The cooler line going from the transmission to its cooler is disconnected and the inlet for the machine goes to the line coming out of the transmission while the outlet from the machine goes to the cooler. Effectively putting the machine in between as an extension of the cooler's line.
The engine of the car is run, causing the transmission's pump to circulate fluid.
The down side to this machine is that it does not effectively remove all of the fluid, since it is connected only at the cooler line and thus often misses much of the fluid that is within the torque converter and other areas of the trans. Rather than replace, it actually dilutes old fluid with new instead. They also do not require the mechanic to remove the transmission pan and replace filters, which means the mechanic is often not looking for problems. This makes this type of flush cheaper, however.
Pump Inlet Flush
This one requires pan removal, which means the filter is often replaced as well. The lines from the machine are connected before and after the transmission's pump at its intake and outlet ports.
Again, the vehicle is run to activate the pump and the new fluid is pushed in through the pump, marshaling the old fluid behind it and out. This replacement method uses about 20 quarts to replace 15, since there is some mixing, but it gets nearly every drop of old fluid out and without putting anything under undue pressure.
How Often To Flush?
This will depend on manufacturer's recommendations. Most fall within the 30,000-45,000 range. A qualified mechanic can do a Cooler flush in about fifteen to twenty minutes while a Pump Inlet flush will require about an hour.