ERISA § 4214 (29 U.S.C. § 1394)

(a) No plan rule or amendment adopted after January 31, 1981, under section 1389 or 1391(c) of this title may be applied without the employer's consent with respect to liability for a withdrawal or partial withdrawal which occurred before the date on which the rule or amendment was adopted.

(b) All plan rules and amendments authorized under this part shall operate and be applied uniformly with respect to each employer, except that special provisions may be made to take into account the creditworthiness of an employer. The plan sponsor shall give notice to all employers who have an obligation to contribute under the plan and to all employee organizations representing employees covered under the plan of any plan rules or amendments adopted pursuant to this section.