For purposes of paragraphs (1)(A)(iii), (1)(B)(iii)(IV), and (2)(A)(iv) of section 401(o) of the Internal Revenue Code of 1986 (as added by this section), a closed class of participants shall be treated as being closed before September 21, 2016, if the plan sponsor's intention to create such closed class is reflected in formal written documents and communicated to participants before such date. (C) Certain post-enactment plan amendments A plan shall not be treated as failing to be eligible for the application of section 401(o)(1)(A), 401(o)(1)(B)(iii), or 401(a)(26) of such Code (as added by this section) to such plan solely because in the case of— (i) such section 401(o)(1)(A), the plan was amended before the date of the enactment of this Act to eliminate 1 or more benefits, rights, or features, and is further amended after such date of enactment to provide such previously eliminated benefits, rights, or features to a closed class of participants, or (ii) such section 401(o)(1)(B)(iii) or section 401(a)(26), the plan was amended before the date of the enactment of this Act to cease all benefit accruals, and is further amended after such date of enactment to provide benefit accruals to a closed class of participants. Any such section shall only apply if the plan otherwise meets the requirements of such section and in applying such section, the date the class of participants is closed shall be the effective date of the later amendment. 206. Modification of PBGC premiums for CSEC plans (a) Flat rate premium Subparagraph (A) of section 4006(a)(3) of the Employee Retirement Income Security Act of 1974 ( 29 U.S.C. 1306(a)(3) ) is amended— (1) in clause (i), by striking plan, and inserting plan other than a CSEC plan (as defined in section 210(f)(1))