ALTER primary key column from INT to BIGINT in production (MySQL 5.6.19a)
If you have enough space, you can create a copy of the actual table and do the work on that:

CREATE TABLE new_tbl [AS] SELECT * FROM orig_tbl;


Then you can change the column as desired:

ALTER TABLE tbl_name MODIFY COLUMN col_name BIGINT AUTO_INCREMENT;


Once the process is done, you can rename the tables:

RENAME TABLE tbl_name TO new_tbl_name, tbl_name2 TO new_tbl_name2;


Then drop the original table, and you should have the spected result.