Hi Lukas!

lukas.php

<?php

/**
 * Temporary function:
 * @return [type] [description]
 */
function convert_to_legacy_posts() {
    if( !is_admin() ) {
        return;
    }

    if( $_REQUEST['convert_to_legacy_posts'] && current_user_can( 'manage_options' ) ) {
        print_r( 'start converting...' );

        $posts = get_posts(
            array(
                'post_type'         => 'XXX',
                'posts_per_page'    => -1
            )
        );

        if( $posts ) {
            foreach( $posts as $post ) {
                update_post_meta( $post->ID, 'XXX', '1' );
            }
        } 
    }
}

add_action( 'admin_init', 'convert_to_legacy_posts' );