4203.3—Plan adoption of special withdrawal rules.
        
        (a) General rule.
         A plan may, subject to the approval of the PBGC, establish by plan amendment special complete or partial withdrawal liability rules. A complete withdrawal liability rule adopted pursuant to this part shall be similar to the rules for the construction and entertainment industries described in  section 4203 (b) and (c) of ERISA. A partial withdrawal liability rule adopted pursuant to this part shall be consistent with the complete withdrawal rule adopted by the plan. A plan amendment adopted under this part may not be put into effect until it is approved by the PBGC.
    
    
        
        (b) Discretionary provisions of the plan amendment.
         A plan amendment adopted pursuant to this part may—
    
    
    
        
        (2) 
         Apply to cessations of the obligation to contribute that occurred prior to the adoption of the amendment.