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. 

const IMergeTreeDataPart & part, DiskType disk_type) const
{
    String best_replica;
    zkutil::ZooKeeperPtr zookeeper = tryGetZooKeeper();
    if (!zookeeper)
        return "";
    Strings zc_zookeeper_paths = getZeroCopyPartPath(*getSettings(), toString(disk_type), getTableSharedID(), part.name,
            zookeeper_path);
    std::set<String> replicas;
    for (const auto & zc_zookeeper_path : zc_zookeeper_paths)
    {
        Strings ids;
        zookeeper->tryGetChildren(zc_zookeeper_path, ids);
        for (const auto & id : ids)
        {
            String zookeeper_part_uniq_node = fs::path(zc_zookeeper_path) / id;
            Strings id_replicas;
            zookeeper->tryGetChildren(zookeeper_part_uniq_node, id_replicas);
            LOG_TRACE(log, "Found zookeper replicas for {}: {}", zookeeper_part_uniq_node, id_replicas.size());
            replicas.insert(id_replicas.begin(), id_replicas.end());
        }
    }
    LOG_TRACE(log, "Found zookeper replicas for part {}: {}", part.name, replicas.size());
    Strings active_replicas;
    
    
    active_replicas.reserve(replicas.size());
    for (const String & replica : replicas)
        if ((replica != replica_name) && (zookeeper->exists(fs::path(zookeeper_path) / "replicas" / replica / "is_active")))
            active_replicas.push_back(replica);