Function group_migration_record_plan 

pub fn group_migration_record_plan(
    direction: GroupMigrationDirection,
    member_count: usize,
) -> GroupMigrationRecordPlan

Select the exact keyed-record actions for one atomic migration batch.

An upgrade tombstones the classic group and writes the full next-gen group. A downgrade does the inverse. Both directions apply the same action to all three per-member next-gen record families.