As the new Blogger is out of Beta, I wanted to relabel my posts as "New Blogger Hacks" and "Old Blogger Hacks". Due to my prior experience, I was worried if all the posts will re-appear in the feed. But as Susan rightly pointed out in a comment, I used the Label Actions dropdown that appears when you goto Posting/Edit posts and thankfully the feed is intact (I was checking the feed once in a while with fast beating heart :)). So, it is safe to update labels like this instead of going to post editor and republishing. Thank you Susan!
(One useful tip: If you want to re-label all the posts under a particular label, it is better to first apply the new label to all of them and then remove the old label. Blogger doesn't have a way to list only unlabelled posts.)