When Alexandra Ansanelli left New York City Ballet for the uncertain stages of Europe this year, the challenge facing her was daunting: to go from being celebrated as one of the ballet world’s premiere “Balanchine-style” dancers to becoming a master of the classic ballet form. Many supposed that the transition would prove impossible. But early reviews (including some from the notoriously tough London press) indicate that Ansanelli may be more versatile than anyone knew.