"The purpose of this revenue procedure is to set forth the procedure by which a plan administrator or plan sponsor may obtain approval of the Secretary of the Treasury for a change in funding method as provided by section 412(c)(5)(A) of the Internal Revenue Code ... [an earlier procedure, Rev. Proc. 2000-40,] provides approval to change the funding method used to determine the minimum funding requirement for defined benefit plans to any one of the methods described therein."