Say a plan is amended as of 1/1/87. It is clear that all benefits accrued up to that time must be protected, thus cannot be reduced. It is also clear that the assumptions and methodology for computing a lump sum must bemaintained for the accd benefit. i.e. if the lump sum were based on 1/1 pbgc rates and an immediate annuity, I presume this must be preserved, but you could use say 1/199 pbgc rates if for eg. ee recd dist in 1999.

Question is can the Plan actually preserve the specific interest rate as of the date of amendment .i.e. the 1/1/87 rate in this case? In other words can they require that this rate be used, so then if the person terminates in 1999 and the rates go down, the lower rate is not required as opposed to using the lower current rate?

Treasury regulation 1.411(d)-4, A-1(B)(1) says: However, such amounts, methods of calculation, or values may be protecetd benefits ... . So, the reference date can be taken as a method of calculation, so a floating interest rate basis can be part of the protected benefit. The change from a floating rate to a fixed rate could be challenged. Also, special rules for a change in the calculation basis sometimes required a better of calculation for a period of time.

However, special relief is available for the transition from PBGC interest rates to GATT interest rates, if one meets specific requirements. So one can change away from the PBGC floating rate.