Some people change the post permalink after publishing the post because they might think that the title of post is not appropriate or they find any good title for the post. Now if you change the permalink of the post then everything looks fine, your new title might makes your post more attractive but somewhere it is wrong.
As you publish a new post and the RSS feed technology update a post in different feed readers of your subscribers. There are many feed readers that watch the title of the post to decide that it is new or not. (Google reader is an example), if you change the title of your post then these feed readers (which work on a particular algorithm) would show your same post twice.
This will make your feed readers confused and the reputation of your blog might effect. So don’t try to do this work.
Make sure that the decided title of an article should be correct and it should tell about the content of an article.
Once the post appear in the feed reader, whatever the change you made on the content of your article it didn’t make any change in your feeds Also you can’t delete your feeds once it is appear in anyone’s feed reader. So be sure whatever you write in an article is true and contains no mistakes.
If you want to change , change your post title, but make sure your permalink is not changed after it, else your blog will start giving 404 page error.
In case you made a big mistake with permalink and there is no option and you have to change it, always add a proper redirection.