Just got doing mine (second time changing the fluid) and installing a Sonnax Zip kit in the valve body, solved approx 70 percent of it's issues but still some lazy downshifts. Your supposed to reset the adaptations but I didn't on purpose as I wanted to see how difference the Sonnax kit made by itself, going to reset the adaptations tomorrow and see if that fixes the rest (I have a 50i with the ZF8HP70 tranny with 136,000 on it). Its not that difficult but you must be meticulous with everything. One misplaced check valve or ball as your tranny is toast. I made videos along the way, if enough interest I'll edit them and post it up. AND BUY THIS:
https://www.amazon.com/gp/aw/d/B0119..._b_pd_title.IT WILL BE THE BEST 100 DOLLARS YOU EVER SPENT. Then when your under your car while it's running watching this amazing device shoot the 4 liters of fluid in the tranny stress free, you can feel sorry for all the poor saps that are trying pump their fluid in, spilling it everywhere and burning themselves on the exhaust....