I do realise that i should be submitting my new sitemap with new url structure but i fear loosing rankings and so i need your advice what to do and how to make sure nothing goes wrong. I have already one site map submitted into google like an year ago.
If you have the old pages there you can do a 301 redirect to the new pages. And do a sitemap of all the pages as they are.
indeed you need to give the new sitemap. as plepco said give a 301 redirect to the old urls and you should be safe. when the new urls are indexed on google, everything will be ok.
yes, you must make a new site if you changed your urls structure. but dont forget to use 301 redirect from old urls to new urls