Blogger Buzz announced (and Kirk spotted even before that, see his post with illustrative pics), Blogger Beta now includes bulk label management facilities so you can add or remove labels from several posts in one go.
The Buzz post even suggests you can label all your old posts easily. Sure you can, but I just wanted to sound a word of warning before you rush off and spend ages labelling all your old posts. Yes, I'm repeating myself, but I think the point merits repeating.
It's worth having a think about why you want labels, and how you want to use them. If the purpose is so that a visitor can click on a label to view all posts in your blog which have the same label, great, go ahead and batch label to your heart's content.
But - but - but - if you want your freshly-labelled old posts to show up as tags on blogosphere search engine Technorati's tag pages, then beware. Labelling old posts just ain't enough for that. Posts will only get picked up on Technorati's tag pages (if at all, given the bugs with indexing tags!) if they are in your blog's feed, i.e. are set to show up on the blog's main page - see this post.