Change author permalink to external URL

You can override the author link via the author_link filter:

function wpd_author_link( $link, $author_id, $author_nicename ){
    return 'http://my.blog.tld/';
}
add_filter( 'author_link', 'wpd_author_link', 20, 3 );

$author_id and $author_nicename are available to the filter so you can fetch whatever data you need to generate the correct link.