I would to be able to re-approve my contributor's edits to an already published post. I.e, when they edit a post - it gets sent back for approval.
Is there a way to do this?
This exact functionality is not available in wordpress core. You may want to completely block them from modifying the published posts. See this capability- http://codex.wordpress.org/Roles_and_Capabilities#edit_published_posts
Otherwise you'll need a plugin