I have submitted site maps on google web master for my web sites, but since i keep changing web pages content after they get cashed .. do i need to resubmit the site maps in such case to make sure google bots crawl those pages or it will happen by default please suggest, what is advisable to make sure pages get quickly re cashed
An XML Sitemap is a source for URLs. If you do not add new URLs or the priority of the URLs is the same in the XML Sitemap then re-scanning it should not cause any changes for the crawler. So no point re-submitting it if you only change content. You might want to show in the XML Sitemap that some pages are updated frequently.
what in case we do changes on the same URL's which are already cashed and are present on the site map already submitted earlier on google webmaster account
In this case you should ping the sitemap to make Google reindex it Here is the sitemap ping URL. Change the sitemap URL to yours http://www.google.com/webmasters/tools/ping?sitemap=http://www.yoursite.com/sitemap.xml
But i will suggest you, to upload your site map when you edit you page, i am not saying that generate your sitemap again and again just edit it manually.
if you are updating your content regularly after caching, you can resubmit it again but its not necessary.