Tchaikovsky’s Nutcracker is one of those works of art about which the public tends to have very strong feelings, and the ballet companies that rely on their annual productions to line their coffers for the year mess with the show at their own risk. This year, the San Francisco Ballet is updating and overhauling its Nutcracker for the first time in nearly two decades, and the hope is that traditionalists won’t be too put off by the changes, innocent and unthreatening as they may be.