I was doing a lot of changes to my old posts, switched to HTTPS, etc. Sometimes it was useful to change some particular text in all my old posts at a time, but there is no such feature in WordPress. But WordPress runs on MySQL and I could use SQL query to update such posts.
Make backup – it's not required but strongly advised 🙂
Now use this query as template to replace in place whatever you need:
mysql> UPDATE wp_posts SET post_content = REPLACE(post_content, '<!--more-->', ''); Query OK, 157 rows affected (0.04 sec) Rows matched: 455 Changed: 157 Warnings: 0
This will remove
<!--more-->from all posts (it's used by WordPress to span article when showed on tag/category pages).
Another example to update all URLs to HTTPS:
UPDATE wp_posts SET post_content = REPLACE(post_content, 'http://gagor.pl', 'https://gagor.pl');
Be careful with that and make DB backup before you start.