Try to put this in functions.php of your theme. It’s untested in this form, but should work.
/**
* Change the Permalink on save / update
*/
function wpse_297006_rewrite_slug( $post_id, $post, $update ){
// check if it's a post
if (!(get_post_type( $post_id ) === 'post')) return;
// check if it's not a revision
if (!(wp_is_post_revision( $post_id ) === false)) return;
// check if it's not a revision
$content = get_post_field('post_content', $post_id);
// set the args
$args = [
'ID' => $post_id,
'post_name' => strtolower( sanitize_title( $content ) ),
];
// unhook this method to prevent infinite loop
remove_action('wp_insert_post', 'wpse_297006_rewrite_slug');
// apply $args when post is saved
wp_update_post( $args );
// re-hook this method
add_action('wp_insert_post', 'wpse_297006_rewrite_slug', 10, 3);
};
add_action('wp_insert_post', 'wpse_297006_rewrite_slug', 10, 3);