Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

std::optional<ReplicatedMergeTreePartHeader> desired_part_header;
            if (!part_znode.empty())
            {
                desired_part_header = ReplicatedMergeTreePartHeader::fromString(part_znode);
            }
            else
            {
                String columns_str;
                String checksums_str;
                if (zookeeper->tryGet(fs::path(part_path) / "columns", columns_str) &&
                    zookeeper->tryGet(fs::path(part_path) / "checksums", checksums_str))
                {
                    desired_part_header = ReplicatedMergeTreePartHeader::fromColumnsAndChecksumsZNodes(columns_str, checksums_str);
                }
                else
                {
                    LOG_INFO(log, "Not checking checksums of part {} with replica {} because part was removed from ZooKeeper", part_name, source_replica_path);
                }
            }
            
            
            
            if (desired_part_header
                && source_part_header.getColumnsHash() == desired_part_header->getColumnsHash()
                && source_part_header.getChecksums() == desired_part_header->getChecksums())
            {
                LOG_TRACE(log, "Found local part {} with the same checksums and columns hash as {}", source_part->name, part_name);
                part_to_clone = source_part;
            }
        }