The chrome casings are separate, so you can replace just those.
I replaced the body-colour casings on a previous A8 I owned with chrome S8-looking casings.
It was several years ago so I'm not sure where I purchased them from, but I do remember they came from Germany, possibly from this seller:
http://www.ebay.co.uk/itm/Audi-A8-S8...-/261189741178