Source

File: wp-includes/class-wp-rewrite.php

public function set_permalink_structure( $permalink_structure ) {
		if ( $permalink_structure != $this->permalink_structure ) {
			$old_permalink_structure = $this->permalink_structure;
			update_option( 'permalink_structure', $permalink_structure );

			$this->init();

			/**
			 * Fires after the permalink structure is updated.
			 *
			 * @since 2.8.0
			 *
			 * @param string $old_permalink_structure The previous permalink structure.
			 * @param string $permalink_structure     The new permalink structure.
			 */
			do_action( 'permalink_structure_changed', $old_permalink_structure, $permalink_structure );
		}
	}