1 or more classes of participants are closed, except that 1 or more classes of participants closed on different dates shall not be aggregated for purposes of determining the date any such class was closed. (C) Highly compensated employee The term highly compensated employee has the meaning given such term in section 414(q). (D) Discriminatory amendments A plan amendment shall be treated as discriminating significantly in favor of highly compensated employees only if such amendment, if adopted before the enactment of this subsection, would have been treated as violating subsection (a)(4). . (b) Participation requirements Paragraph (26) of section 401(a) of the Internal Revenue Code of 1986 is amended by adding at the end the following new subparagraph: (I) Protected participants (i) In general A plan shall be deemed to satisfy the requirements of subparagraph (A) if— (I) the plan is amended— (aa) to cease all benefit accruals, or (bb) to provide future benefit accruals only to a closed class of participants, (II) the plan satisfies subparagraph (A) (without regard to this subparagraph) as of the effective date of the amendment, and (III) the amendment was adopted before April 5, 2017, or the plan is described in clause (ii). (ii) Plans described A plan is described in this clause if the plan would be described in subsection (o)(1)(C), as applied for purposes of subsection (o)(1)(B)(iii)(IV) and by treating the effective date of the amendment as the date the class was closed for purposes of subsection (o)(1)(C). (iii) Special rules For purposes of clause (i)(II), in applying section 410(b)(6)(C), the amendments described in clause (i) shall not be treated as a significant change in coverage under section 410(b)(6)(C)(i)(II). (iv) Spun-off plans For purposes of this subparagraph, if a portion of a plan described in clause (i) is spun off to another employer, the treatment under clause (i) of the spun-off plan shall continue with respect to the other employer. . (c) Effective date (1) In general Except as provided