Had a Grand Prix here today. The owner complained of harsh shifts. So he dropped the car off and left me to it. I dropped the pan and drained the fluid. There was a fair amount of build up on the magnet. So his clutches are a bit worn. I removed his shift accumulator and opened it up. I then removed both pistons. Neither came out easy. I then wiped down the cylinders and looked them over. Both were very smooth and shinny. Well, that just don't do. So I took some 400 grit wet sand sandpaper and scuffed up the cylinder walls, just a little. Cleaned up the piston O-rings and put everything back together. Installed a new filter and torqued the pan back on. Filled it up and went for a drive. Very smooth shifts. Another happy GP driver.
So, remember, if you have harsh shifts, chances are, it'* a easy fix.