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 String & source_replica,
    const String & new_part_name,
    const DiskPtr & disk,
    const String & path)
{
    if (source_replica.empty())
    {
        LOG_INFO(log, "No active replica has part {} on shared storage.", new_part_name);
        return nullptr;
    }
    const auto storage_settings_ptr = getSettings();
    auto metadata_snapshot = getInMemoryMetadataPtr();
    try
    {
        return fetchExistsPart(new_part_name, metadata_snapshot, fs::path(zookeeper_path) / "replicas" / source_replica, disk, path);
    }
    catch (Exception & e)
    {
        if (e.code() == ErrorCodes::RECEIVED_ERROR_TOO_MANY_REQUESTS)
            e.addMessage("Too busy replica. Will try later.");
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        throw;
    }
}
void StorageReplicatedMergeTree::executeDropRange(const LogEntry & entry)