Change author permalink using htaccess?

it’s simpler to do this via your theme’s functions.php or a plugin rather than .htaccess:

function wpse31040_author_rewrite(){
    add_rewrite_rule(
        'author/([0-9]+)/?$',
        'index.php?author=$matches[1]',
        'top' );
}
add_action( 'init', 'wpse31040_author_rewrite' );

make sure to visit the permalinks settings page after adding this to flush rewrite rules.