The IRS has provided a voluntary correction program (VCP) Submission Kit for plan sponsors who failed to adopt a new Pre-Approved Defined Contribution Plan by the April 30, 2016 deadline.
The following is an excerpt from the IRS notification
“Sponsors of pre-approved defined contribution retirement plans were generally required to sign new plan documents, generally on or before April 30, 2016, that incorporated changes required by the Pension Protection Act (PPA). Defined contribution plans include profit-sharing plans, 401(k) plans, and money purchase pension plans. Pre-approved plans are plan documents that have been approved by the IRS and are sold to plan sponsors through law firms, banks, brokers, other financial institutions or plan administrators. The IRS issued favorable opinion letters approving these plans to the authors of these pre-approved plan documents in March of 2014 (or shortly after March 31, 2014).
Some plan sponsors have a later adoption deadline. If a plan sponsor wishes to convert an existing individually designed plan into a current defined contribution pre-approved plan they have until April 30, 2017 to adopt the pre-approved defined contribution plan.”
Details, including the required documentation and procedures, are available here