MySQL 8.0

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CREATE TABLE wp_posts (post_content varchar(29))
INSERT INTO wp_posts VALUES('a»<br />b')
UPDATE wp_posts SET post_content = REPLACE(post_content, '»<br />', '»\\n\\n')

Rows matched: 1  Changed: 1  Warnings: 0

SELECT * FROM wp_posts

post_content

a»\n\nb