Please...dear God don't scrap your website (especially if you are ranking well for it). Even if the content is completely different, figure out where to redirect your legacy pages to and use 301 redirects. Just make sure that you are doing one to one redirects to maintain as much of your link equity as possible. Sorry for such a short and simple response to your long question, but this only required a simple answer. If you are offering the same products or services create a massive .htaccess file and redirect the hell out of your old site.
You owe me a Coke.