RSS Feed question?

Hey guys, do you know of any way to prevent older posts from being updated, when you run a new syndicated feed?

For example : I run FeedWordpress to get new blog posts from affiliate feeds and when I do it alters the older posts.See I sometimes like to add a bigger pic or change a misspelling in a blog post.So when I run FeedWordpress, it changes all the older posts back to the original way they were before my editing. :rolleyes: