Replaced Speed Sensor - Still have Problem of Shifting Up and Down

It is most likely something other than the torque converter. The torque converter would be constant slipping, not just when shifting. I would check the data from trans mission control unit, or have a shop check it. I have seen lots of Audi/VW cars have shifting problems with old fluid. When was the last in me you serviced your trans and replaced the filter? There is a trans filter kit available for your A8.