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 (config->has("zookeeper"))
                    global_context->reloadZooKeeperIfChanged(config);
                global_context->reloadAuxiliaryZooKeepersConfigIfChanged(config);
                std::lock_guard lock(servers_lock);
                updateServers(*config, server_pool, async_metrics, servers);
            }
            global_context->updateStorageConfiguration(*config);
            global_context->updateInterserverCredentials(*config);
            CompressionCodecEncrypted::Configuration::instance().tryLoad(*config, "encryption_codecs");
#if USE_SSL
            CertificateReloader::instance().tryLoad(*config);
#endif
            ProfileEvents::increment(ProfileEvents::MainConfigLoads);
        },
         false);  
    const auto listen_hosts = getListenHosts(config());
    const auto interserver_listen_hosts = getInterserverListenHosts(config());
    const auto listen_try = getListenTry(config());
    if (config().has("keeper_server"))
    {
#if USE_NURAFT
        
        
        bool can_initialize_keeper_async = false;
        if (has_zookeeper) 
        {
            
            
            can_initialize_keeper_async = global_context->tryCheckClientConnectionToMyKeeperCluster();