Batch edit post categories

I’m a WP noob who just moved my blogger blog over to WP (imported via wordpress.com — a nice hack for those of us coming from the new blogger.com world).

One WP feature I really want to exploit is categories. Is there any “batch” way I can edit the categories assigned to more than one post at a time? I do not want to have to open all 500+ old post and assign categories one post at a time.

Are there related tools for managing and assigning tags on more than one post at a time? I might be able to use tags instead of categories, if need be.

I tried searching for similar support questions, or for related plugins, but constructing a query where ‘category’ is the keyword yields lots of spurious hits… sorry if this question is an old one…