Moving and changing a large site and not upsetting the cart

We built a large musical resource directory on someone else's platform... since that time we have developed the capacity to build our own directories (Joomla). We are in the process of recreating it on our own servers. Soon we will want to swap it out...our directory on our servers. Static pages to dynamic. Though we're live about six months we're very happy with the steady progressive growth in our Google rankings...so we don't want to upset the cart. The skinny- we want to move the URL to us and replace the old pages with the new (99%) identical pages AND keep the Google thing cooking. Note: The original is a proprietary platform and we can't just shift it to us first. We're marketers who are SEO newbies... any insight would be appreciated.

Thanks

You'll need to create 301 permanent redirects from each old page to its equivalent new page. The 301s can be generated through a .htaccess file if the old site uses an apache server, or through php headers if it uses php, or through asp headers if using .asp

The 301 redirect tells Google that the page has permanently moved to the new location, and any existing back links are also transferred over to the new pages.