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. 

if (!added_parts_str.empty())
        {
            ReplicatedMergeTreeQuorumAddedParts part_with_quorum(format_version);
            part_with_quorum.fromString(added_parts_str);
            auto added_parts = part_with_quorum.added_parts;
            for (const auto & added_part : added_parts)
                if (!getActiveContainingPart(added_part.second))
                    throw Exception(
                        "Replica doesn't have part " + added_part.second
                            + " which was successfully written to quorum of other replicas."
                              " Send query to another replica or disable 'select_sequential_consistency' setting.",
                        ErrorCodes::REPLICA_IS_NOT_IN_QUORUM);
            for (const auto & max_block : part_with_quorum.getMaxInsertedBlocks())
                max_added_blocks[max_block.first] = max_block.second;
        }
    }
    return max_added_blocks;
}
void StorageReplicatedMergeTree::read(
    QueryPlan & query_plan,
    const Names & column_names,
    const StorageSnapshotPtr & storage_snapshot,
    SelectQueryInfo & query_info,
    ContextPtr local_context,
    QueryProcessingStage::Enum processed_stage,
    const size_t max_block_size,
    const unsigned num_streams)
{
    
    const bool enable_parallel_reading = local_context->getClientInfo().collaborate_with_initiator;