When I change a slug from post-a to post-b, will WordPress create a redirect from /post-a to /post-b, like HTTP 301 or something similar? If not, is there a plugin for that?
I just went and tested this (WP 3.3.1), the wordpress core will automatically do a redirect if you change the post slug, even if you change the slug from post-a to post-b and then to post-c, post-a will return a 301 to post-c. That said, the redirection plugin that Mike Jolley recommended is an outstanding plugin and a great way to monitor for 404 errors and such.