My thought, and reason for asking, is that by idling the engine with a cooler line disconnected, you can pump the fluid out using the transmission's own pump.
Roughly, connect a clear plastic tube to the cooler line, place in a container on which you have marked a 2 QT line. Idle the engine until 2 qts have been pumped out. Shut it off, refill, and repeat until the fluid is nice and clear.
This has the advantage of getting all the fluid out (including the convertor). And you would have to refill through that hex bolt (instead of through the dipstick tube).
On yours though, sheesh...I have never dealt with one of these...drain and refill I guess, but I would want to know if the refill level (hex bolt) was for cold or hot...hot, I would imagine...
My BIL is a GM powertrain engineer...they design them this way because in many of the transmissions that fail, the analysis shows contamination....so, the larger threat to the trans is not wear or fluid breakdown...it's maintenance! The largest contaminant? Rag lint...red in color...shop rags...who knew?
I am with you - they want to get the trans through warranty without failures - but we want it to last longer...
