I am about to upload a revamped site of Joomla (update from 1.5 to 2.5 + changes). I currently have a test bed subdomain that I am currently working on. In few days I am about to do the swap and replace the old site with the new one.

I am worrying about Search Engines and specifically Google. The site currently has a very good rank (appears 2nd in the search), what actions do I have to take in order to be updated and preserve the rank? (except submitting the new sitemap I guess).

It's not a difficult task but because I don't have the option to be wrong or mistakes to be done I an asking for a more "expert" advice.