Blogging engines don't allow a quick way to change categories, or to add tags to many posts at the same time, so managing your old posts can be a lengthy process.
A friend of mine, Marco De Sanctis, had to add tags to all his old posts and to reorganize the categories of his blog, so he built a small tool that retrieves all the posts via MetaWeblog API, and allow to select many posts, and add tags or categories in batch.
More information about this wonderful tool, named "Blog Manager", on Marco's blog: A tool to manage your blog categories and Technorati Tags.
Blog Manager is available both in Italian and English, and support for other languages is coming soon.
You can download the tool and also the code (with the solution in VS2008 format) is available.