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. 

for (const auto & replicas : src_cluster->getShardsAddresses())
    {
        
        for (const auto & node : replicas)
        {
            auto connection = std::make_shared<Connection>(
                node.host_name, node.port, query_context->getGlobalContext()->getCurrentDatabase(),
                node.user, node.password, node.quota_key, node.cluster, node.cluster_secret,
                "ParallelInsertSelectInititiator",
                node.compression,
                node.secure
            );
            auto remote_query_executor = std::make_shared<RemoteQueryExecutor>(
                connection,
                query_str,
                Block{},
                query_context,
                nullptr,
                Scalars{},
                Tables{},
                QueryProcessingStage::Complete,
                extension);
            QueryPipeline remote_pipeline(std::make_shared<RemoteSource>(remote_query_executor, false, settings.async_socket_for_remote));
            remote_pipeline.complete(std::make_shared<EmptySink>(remote_query_executor->getHeader()));
            pipeline.addCompletedPipeline(std::move(remote_pipeline));
        }
    }
    return pipeline;
}
std::optional<QueryPipeline> StorageReplicatedMergeTree::distributedWrite(const ASTInsertQuery & query, ContextPtr local_context)