Who has fixed this issue with the harsh down shifting?
Purely a guess on my part. The transmission is most likely computer controlled, you may have to go to a GM dealer to have it properly diagnosed. An independent MIGHT have the correct software to diagnose it. One possibility is the lockup torque converter.
As far as your question, other info is needed, is the downshift during rapid acceleration such as passing or when it shifts to first gear at a complete stop?