A new Broadway collaboration teaming choreographer Twyla Tharp with the music of Bob Dylan is going through more than the usual share of pre-opening night growing pains. Several cast members have been fired over the last few weeks, and now, the show’s female lead has been replaced by her understudy. “The show is in such flux, theater writers have repeatedly been asked to postpone their visits. Even cast members aren’t always clear about what’s going on.”