Treas. Reg. § 1.411(a)-8T

(1) In general. Under section 411(a)(10)(B), for plan years for which section 411 applies, if the vesting schedule of a plan is amended, the plan will not be treated as meeting the minimum vesting standards of section 411(a)(2) unless the plan as amended provides that each participant whose nonforfeitable percentage of his accrued benefit derived from employer contributions is determined under such schedule, and who has completed at least 3 years of service with the employer, may elect, during the election period, to have the nonforfeitable percentage of his accrued benefit derived from employer contributions determined without regard to such amendment. Notwithstanding the preceding sentence, no election need be provided for any participant whose nonforfeitable percentage under the plan, as amended, at any time cannot be less than such percentage determined without regard to such amendment. For employees not described in § 1.411(a)-3T(e)(1), this section shall be applied by substituting “5 years of service” for “3 years of service” where such language appears.

(2) Election period. For purposes of subparagraph (1) of this paragraph, the election period under the plan must begin no later than the date the plan amendment is adopted and end no earlier than the latest of the following dates:

(3) Service requirement. For purposes of subparagraph (1) of this paragraph, a participant shall be considered to have completed 3 years of service if such participant has completed 3 years of service, whether or not consecutive, without regard to the exceptions of section 411(a)(4) prior to the expiration of the election period described in subparagraph (2) of this paragraph. For the meaning of the term “year of service”, see regulations prescribed by the Secretary of Labor under 29 CFR Part 2530, relating to minimum standards for employee pension benefit plans.