Condition display metabox in case the post is saved

As an alternative to @m0r7if3r’s solution, the add_meta_boxes hook optionally passes two variables, the post type and post object. You can use this to conditionally add your metabox. New posts have the post status of ‘auto-draft’.

add_action( 'add_meta_boxes', 'myplugin_add_custom_box',10,2);
function myplugin_add_custom_box($post_type, $post) {
    if($post->post_status != 'auto-draft'){
        add_meta_box( 
         'myplugin_sectionid',
         __( 'My Post Section Title', 'myplugin_textdomain' ),
         'myplugin_inner_custom_box',
         'post' );
    }
}