BatchMove is a WordPress plugin that make it possible to change the category where a post(s) belong to. It is a rewrite of the plugin Rob Miller made, it uses a class for its operations and all parts are separated. You can Add, Update or Delete posts categories. First you make a selection of the records you want to change, then choose new category and the action. Thats all, send your request and the changes are made, the program send you direct to choosen action category. We have made BatchMove for WP users who maintain large WordPress implementations. The exact use is to move/ a lot of posts to an ohter category, for example all post from news category who from for 1 january to archive archive->2011 or category any name you want. Or just what you want, the different is that this routine can update/add/delete and not just add a post to a category.
Perhaps the best of this plugin, you are able to select a date range!