Did you set up a 301 redirect for your old site? 301 redirects send everyone to your new pages - and tell the search engines that they pages have been moved.
That's a good tip - although I don't think it guarantees that every one of your pages will be indexed, it does help the search engine to at least know about the pages and navigate to them (just in case you have pages that are not linked to by any other pages on your site).Basically, it's an XML format 'map' of your forum that you submit to a search engine. It means the search engine will index all your pages.
yes this is the easiest way for your site to get re-indexed by Google. Submitting your site to google submit page will take you weeks or even months.Bookmarking the site somewhere like Delicious or Digg should help. These sites are spidered countless times per day after all.