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. 

Poco::Logger * logger, MergeTreeSettingsPtr table_settings)
{
    if (zookeeper->expired())
        throw Exception("Table was not dropped because ZooKeeper session has expired.", ErrorCodes::TABLE_WAS_NOT_DROPPED);
    auto remote_replica_path = zookeeper_path + "/replicas/" + replica;
    LOG_INFO(logger, "Removing replica {}, marking it as lost", remote_replica_path);
    
    
    zookeeper->trySet(remote_replica_path + "/is_lost", "1");
    
    
    if (!zookeeper->exists(remote_replica_path))
    {
        LOG_INFO(logger, "Removing replica {} does not exist", remote_replica_path);
        return;
    }