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 (disk->supportZeroCopyReplication())
    {
        if (is_freezed)
        {
            FreezeMetaData meta;
            if (meta.load(disk, path))
            {
                FreezeMetaData::clean(disk, path);
                return removeSharedDetachedPart(disk, path, part_name, meta.table_shared_id, meta.zookeeper_name, meta.replica_name, "", getContext());
            }
        }
        else
        {
            String table_id = getTableSharedID();
            return removeSharedDetachedPart(disk, path, part_name, table_id, zookeeper_name, replica_name, zookeeper_path, getContext());
        }
    }
    disk->removeRecursive(path);
    return false;
}
bool StorageReplicatedMergeTree::removeSharedDetachedPart(DiskPtr disk, const String & path, const String & part_name, const String & table_uuid,