29 C.F.R. § 4281.33

(a) General. The plan sponsor of a plan that has been amended to reduce benefits under this subpart shall amend the plan to restore those benefits before adopting any amendment increasing benefits under the plan. A plan is not required to make retroactive benefit payments with respect to any benefit that was reduced and subsequently restored in accordance with this section.

(b) Notice to the PBGC. The plan sponsor shall notify the PBGC in writing of any restoration under this section. The notice shall include the information specified in § 4281.32 (d)(1) through (d)(4); a statement that a plan amendment restoring benefits has been adopted, the date of adoption, and the effective date of the amendment; and a certification, signed by the plan sponsor or its duly authorized representative, that the amendment has been adopted in accordance with this section.