Problem with modifing tables with STORED columns used in procedures

CREATE TABLE `A` (

  `AC1` int(11) PRIMARY KEY,

  `U_ID` int(11) NOT NULL,

  `U_N` int(11) GENERATED ALWAYS AS (`U_ID`) STORED

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE IF NOT EXISTS B(

  B_ID int(11) AUTO_INCREMENT PRIMARY KEY,

  BC1 int(11) NOT NULL

);

DELIMITER ;;

CREATE OR REPLACE PROCEDURE proc_migrate()

BEGIN

  DECLARE done INT DEFAULT 0;

  DECLARE p_AC1 int(11);

  DECLARE cur1 CURSOR FOR SELECT AC1 FROM A BU;

  DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET done = 1;

  OPEN cur1;

  REPEAT

    FETCH cur1 INTO p_AC1;

    IF NOT done THEN

      BEGIN

        INSERT INTO B (BC1) VALUES(p_AC1);

      END;

    END IF;

  UNTIL done END REPEAT;

  CLOSE cur1;

END;;

DELIMITER ;

CALL proc_migrate();

ALTER TABLE A MODIFY COLUMN AC1 int(11) NOT NULL;