/**
 * Redirect a "/%postname%/" permalink structure to the their new URL.
 * 
 * @link http://wordpress.stackexchange.com/q/136306/1685
 */
function wpse_136306_redirect() {
    global $wp;

    // Check that we\'ve hit a 404 for a non-empty permalink without slashes
    if ( is_404() && strlen( $wp->request ) && strpos( $wp->request, \'/\' ) === false ) {
        // Attempt to find a post that matches the request.
        $posts = get_posts(
            array(
                \'posts_per_page\' => 1,
                \'name\' => $wp->request,
            )
        );

        if ( $posts && $url = get_permalink( $posts[0]->ID ) ) {
            // All good, let\'s redirect to the new URL.
            wp_redirect( $url, 301 );
            exit;
        }
    }
}

add_action( \'template_redirect\', \'wpse_136306_redirect\', 1 );

如果您对Wordpress插件没有问题，可以使用WP redirection plugin.

正则表达式是您的朋友https://redirection.me/support/redirect-regular-expressions/

在Source URL, 设置.*

在里面Target URL, 设置/category/$1 . 棘手的部分是你的category 是一个变量。