From a cutback perspective, there is no protected right to continued participation, only to benefits already accrued. So if you have eligibility that less than the statutory maximum, an amendment could make a current participant ineligible for continued participation until new eligibility is met. Some documents default to grandfathering current participants, others it would depend on the amendment.
You would still have to make sure that the amendment itself isn't discriminatory.
EOB (Ch 2, Section VI, Part E)