If you're using Apache, use 301 redirects in your .htaccess file to force a 'permanent redirect' from the old URLs to the new ones. This will indicate to Google, and the other search engines, to update their records, and let them know that the page has simply moved.
You need to upload an .htaccess file that looks something like this:
Hemanth, the robots.txt file (which, I guess, certain hosting companies let you modify from cPanel, or you can just upload the darn thing yourself) is a list of web pages you want to tell the search engines to ignore and not index.
But, yeah, submitting a revised sitemap certainly won't hurt.
I am not agreeing with your point. Robots.txt file is not only for indexing, it also helps to crawl your WebPages. So that, if you update robots.txt file, any search engine can identifies and crawls your updated pages automatically.