"This letter is ... a ruling on the effect of an amendment changing the interest rate lookback month with respect to preconversion benefits under section 411(b)(5) ... and the regulations thereunder, following the conversion of a defined benefit pension plan ... to a cash balance plan that is intended to be a statutory hybrid plan under section 411(a)(13).... [P]resent value determinations [prior to conversion] were based on interest rates for the lookback month that is the first full calendar month preceding the Plan Year in which the annuity starting date occurs.... [Under the plan amendment, for any distribution having an annuity starting date after the year in which the conversion occurred,] the Look-back Month [became] the month of September in the Plan Year preceding the Plan Year in which the annuity starting date occurs."