The NCAA inches closer every day to a tipping point of dramatic overhaul. Years of tectonic shifts around college sports could soon usher in an era its leaders and administrators have long tried to avoid: the treatment of college athletes as employees. The next milestone could come Tuesday, when the Dartmouth men’s basketball team will vote on whether to form a union. The university is countering by fighting a National Labor Relations Board regional director’s finding that the basketball players are employees and entitled to union representation, but the effort is just one of several concurrent legal battles challenging the bedrock principle of amateurism that the NCAA has long prided itself on maintaining.