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. 

LOG_TRACE(log, fmt::runtime(message));
        return handle_noop(message);
    };
    bool assigned = false;
    if (!partition && final)
    {
        DataPartsVector data_parts = getVisibleDataPartsVector(query_context);
        std::unordered_set<String> partition_ids;
        for (const DataPartPtr & part : data_parts)
            partition_ids.emplace(part->info.partition_id);
        for (const String & partition_id : partition_ids)
        {
            assigned = try_assign_merge(partition_id);
            if (!assigned)
                break;
        }
    }
    else
    {
        String partition_id;
        if (partition)
            partition_id = getPartitionIDFromQuery(partition, query_context);
        assigned = try_assign_merge(partition_id);
    }
    table_lock.reset();
    for (auto & merge_entry : merge_entries)
        waitForLogEntryToBeProcessedIfNecessary(merge_entry, query_context);
    return assigned;
}
bool StorageReplicatedMergeTree::executeMetadataAlter(const StorageReplicatedMergeTree::LogEntry & entry)