Answer by Susang for How to change primary key's data type from int to bigint in a huge transactional database under tremendous load?

You can use this technique, first create new table as following with the conversion of datatype by copying first table structure:

SELECT id, CAST(test_id AS BIGINT) test_id INTO test_new  FROM test

Then verify the data in new table and remove old one and again rename new table name as old one.