Asset managers and trading desks will be scrambling next month to account for the market juggernaut

Additions and subtractions to the S&P 500 are normally a ho-hum affair. The 509th biggest company in the U.S. might jump to 497th place, and thus into the index. Investors who track it buy the one stock and sell another.
But no one has ever tried to add Tesla Inc., a $555 billion company prone to huge swings in price. Thats happening next month, and its causing headaches across Wall Street.
To…