(a) General rule. Subject to the approval of the PBGC pursuant to this subpart, a plan, other than a plan that primarily covers employees in the building and construction industry, may adopt, by amendment, any allocation method or modification to an allocation method that is not permitted under subpart B of this part.

(b) Building and construction industry plans. Subject to the approval of the PBGC pursuant to this subpart, a plan that primarily covers employees in the building and construction industry may adopt, by amendment, any allocation method or modification to an allocation method that is not permitted under section 4211 of ERISA if the method or modification is applicable only to its employers that are not construction industry employers within the meaning of section 4203(b)(1)(A) of ERISA.

(c) Substantial overallocation not allowed. No plan may adopt an allocation method or modification to an allocation method that results in a systematic and substantial overallocation of the plan's unfunded vested benefits.

(d) Use of method prior to approval. A plan may implement an alternative allocation method or modification to an allocation method that requires PBGC approval before that approval is given. However, the plan sponsor shall assess liability in accordance with this paragraph.

(1) Demand for payment. Until the PBGC approves the allocation method or modification, a plan may not demand withdrawal liability under section 4219 of ERISA in an amount that exceeds the lesser of the amount calculated under the amendment or the amount calculated under the allocation method that the plan would be required to use if the PBGC did not approve the amendment. The plan must inform each withdrawing employer of both amounts and explain that the higher amount may become payable depending on the PBGC's decision on the amendment.

(2) Adjustment of liability. When necessary because of the PBGC decision on the amendment, the plan shall adjust the amount demanded from each employer under paragraph (c)(1) of this section and the employer's withdrawal liability payment schedule. The length of the payment schedule shall be increased, as necessary. The plan shall notify each affected employer of the adjusted liability and payment schedule and shall collect the adjusted amount in accordance with the adjusted schedule.