I have a DB plan sponsored by a governmental unit (a hospital owned by a county). The plan is safe harbor, uses a volume submitter document, and is pretty much vanilla.

The sposor adopted the GATT provisions (mortality table and interest rate) for determining lump sums 3 years ago. Two years ago, the plan was frozen, in anticipation of terminating when they have enough money. They don't have enough; with the recent decline in 30-year treasury rates, the underfunding has gotten worse.

Can the plan be amended to change the actuarial equivalent definition (currently GATT basis) to something less expensive (such as an interest rate of 7.5%) ?

Currently, the plan contains fairly common language stating that "an amendment shall not not reduce the vested benefit of a participant determined as of the later of the adoption date or the amendment's effective date." (Note that that sentence does not refer to the defined term "Accrued Benefit".)

Federal law would not impose any obstacles to such a change. However, you need to look carefully at the law and court decisions in your own state. In particular, courts in about two-thirds of all states have held that routine state constitutional provisions on nonimpairment of contracts prevent any cutbacks even in future benefit accruals for existing employees, much less already accrued benefits.