Why are proper, tight QDROs so hard to find? Any decent QDRO will say what to do if the AP dies first before commencement, if the AP dies first after commencement, if the participant dies first before commencement or if the participant dies first after commencement. Not rocket science!
Such an amendment would not be possible at all. A lump sum is a protected right and feature of the accrued benefit and cannot be removed.
The best you can do is remove the lump sum option for future accruals, but you can't take it away from benefits already earned. Therefore, it would be effective for benefits earned after the effective date of the amendment.