they can be polished back up just like any painted surface.. Can start at 2000 grit wet sand or a pretty tough polish/compound and see what happens..

I think hammer and weed are onto something.. A lot of people skip over the headlights when they do wax/polish. I treat them just like a piece of painted metal.. wax goes on over them..

Mine havn't fogged or anything.. although there are a few bug marks and nicks from road debris