The National Labor Relations Board has reset its standard for determining whether a proposed voting unit is appropriate for collective bargaining under federal labor law. Employers and unions now will have to adapt quickly to a changing landscape following the board’s decision in PCC Structurals, Inc.

Setting the clock back more than six years, the Republican-majority labor board overruled the 2011 decision in Specialty Healthcare and returned to a traditional community-of-interest standard for determining an appropriate bargaining unit in each...