Multiple Category Change

Try the Batch Category Editing for WordPress and Batch Categories plugins.

This is one of the more popular Ideas that might be implemented (maybe). See: Batch Categories

Brilliant
https://robm.me.uk/projects/plugins/wordpress/batch-categories

Both of these plugins are pretty old. Batch Categories no longer seems to work. Is there a current plugin for this problem?

Bulk Edit Posts is now part of WordPress (no plugin required) as of WP 2.7

Goto the Edit Posts page, click the checkbox next to each of the posts you want to edit as a group, then click Edit from the pulldown at the top left of the page. Click the Apply button next to the pulldown and you should see the Bulk Edit Posts section appear at the top of the post list. Make your changes and click the Update Posts button to apply the changes to all of the selected posts.