Modify arguments for parent theme’s `wp_register_style` via child theme

I don’t think there’s a built-in safe way to do this, but you could directly modify the $wp_styles global after things are registered but before they’re output, or at least use it to fetch the parameters a style was originally registered with.

global $wp_styles;
if( isset( $wp_styles->registered['a_stylesheet'] ) ){
    $wp_styles->registered['a_stylesheet']->args="handheld";
}

Not ideal, but it may work for you.