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. 

{
    String table_name = table_id.table_name;
    
    String table_metadata_path = getObjectMetadataPath(table_name);
    String table_metadata_tmp_path = table_metadata_path + ".tmp";
    String statement;
    {
        ReadBufferFromFile in(table_metadata_path, METADATA_FILE_BUFFER_SIZE);
        readStringUntilEOF(statement, in);
    }
    ParserCreateQuery parser;
    ASTPtr ast = parseQuery(
        parser,
        statement.data(),
        statement.data() + statement.size(),
        "in file " + table_metadata_path,
        0,