I was interested in adding the bunk to a 19' CCD that didn't come from the factory with the option (was contemplating changing from my 16' to the 19'). I emailed with a rep from Airstream/Jackson Center, and he pointed out a couple of things. First, there are different ribs or bracing inside the trailer wall that are added to take the extra weight, and these are not included in the trailers that don't come with the bunk option. Second, there are differences in the window - the factory trailers with the bunk option don't have a window that opens, said it interferes with the bunk.
Bottom line was that it was not easy to add the bunk later.
This was a couple of years ago, so would be a good idea for you to 'refresh' this info with Airstream before deciding- will be interesting to see if you get the same info!