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 (!skip_startup_tables)
    {
        
        startupTables(pool, mode);
    }
}
void DatabaseOrdinary::loadTablesMetadata(ContextPtr local_context, ParsedTablesMetadata & metadata, bool is_startup)
{
    size_t prev_tables_count = metadata.parsed_tables.size();
    size_t prev_total_dictionaries = metadata.total_dictionaries;
    auto process_metadata = [&metadata, is_startup, this](const String & file_name)
    {
        fs::path path(getMetadataPath());
        fs::path file_path(file_name);
        fs::path full_path = path / file_path;
        try
        {
            auto ast = parseQueryFromMetadata(log, getContext(), full_path.string(),  true,  false);
            if (ast)
            {
                auto * create_query = ast->as<ASTCreateQuery>();
                
                create_query->setDatabase(TSA_SUPPRESS_WARNING_FOR_READ(database_name));
                
                if (create_query->uuid != UUIDHelpers::Nil)
                {
                    
                    if (is_startup)
                    {