Hey friends i would like to know the problem that would come when a URL is changed and how it would be impacted to the Google Sitemap Exchange.. And also what are the solutions to overcome those impacts.....
if the url is changed you can make 301 redirect in htaccess so that the old url is directed to the new one. url in sitemap is kept unchanged.
If you change a URL then update the sitemap and remove it from there and then resubmit the sitemap to the search engines. Even if you do all this the bots will most likely hang on to the URL for some time. Redirect is best as already mentioned but even if it will return a 404 it will eventually be dropped by the bot after a few retries.