So you want to replace your old website with new one that has a totally different link structure and you don’t want to loose your existing visitors. How you can do that ?? well, you can do that by redirecting your visitor who visit not found pages to index page of your website using .htaccess.
To redirect all pages that not found (because it has been removed) to certain page in this case is to index page of your website, you can do that by simply adding this code: ErrorDocument 404 /index.html to your .htaccess file.