Once "burned," what happens to my original feed?

Nothing.

It stays where it is, and FeedBurner checks for updates approximately every 30 minutes. When FeedBurner detects changes in your original feed, those changes are read into FeedBurner and the updates are passed through to the FeedBurner "burned" version of your syndicated content in the ways you've specified. In a sense, the FeedBurner version of your feed becomes the public view of your syndicated content, and your original feed becomes a private feed that only you and FeedBurner know about.