MySQL 8.0

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SELECT VERSION();

VERSION()

8.0.30

SELECT REGEXP_REPLACE('567890', '[7]', 'z');

REGEXP_REPLACE('567890', '[7]', 'z')

56z890

DROP TABLE IF EXISTS `test`;
CREATE TABLE IF NOT EXISTS `test` (
  `field` TEXT
);
INSERT INTO `test` (`field`)
VALUES ('567890');
UPDATE `test`
SET `field` = REGEXP_REPLACE(`field`, '[7]', 'z');

Rows matched: 1  Changed: 1  Warnings: 0

SELECT `field`
FROM `test`;

field

56z890

UPDATE `test`
SET `field` = '567890';

Rows matched: 1  Changed: 1  Warnings: 0

SELECT `field`
FROM `test`;

field

567890

UPDATE `test`
SET `field` = CAST(REGEXP_REPLACE(`field`, '[7]', 'z') AS CHAR);

Rows matched: 1  Changed: 1  Warnings: 0

SELECT `field`
FROM `test`;

field

56z890