The Statutory requirement only exists for MDAPs.

The current regulatory requirement is in DoDI 5000.88 Engineering of Defense Systems

3.5 PROGRAM TECHNICAL REVIEWS AND ASSESSMENTS.

   a. Technical Reviews.

   ...

    (2) Unless waived through the SEP approval process, the PM will conduct these system level reviews, or equivalent:

       (a) System requirements review or system functional review.

       (b) Preliminary design review (PDR).

     ...

So if your Systems Engineering Plan (or equivalent document) only called for a CDR-equivalent review, there is no mandatory requirement to go back and hold a PDR.