Only a confused holder would sell. In fact, the stock should continues running up to the day of effectiveness. Then from Sep 1st, the stock may fall a little, consolidate and eventually start moving upward again as new investors think they are buying the stock at low prices.
And if news of inclusion into the S&P come before Battery day on 22nd September, the stock will run up crazily.
May run up to $600 by early October assuming a price of $400 on September 1st.