WordPress – Update all existing posts at once

Question

I’m not sure if it will work, but you could try running wp_update_post() on all posts concerned. It will trigger the save_post hook.

According to the Codex:

Unlike wp_insert_post(), it is only necessary to pass the ID of the
post to be updated and the elements to be updated.

You could try looping an array of post IDs and running the function on them. You can fetch the post IDs using WP_Query.

foreach( $posts_id as $post_id ){
    wp_update_post( $post_id );
}