I'm in the midst of moving a website from mediawiki to a bespoke solution for hosting the data which will enforce structure on what's being presented. In the process, URLs will change, but, part of the migration is setting things up so that, for example, if someone goes to http://www.rejectionwiki.com/index.php?title=Acumen they will be redirected automatically to http://www.rejectionwiki.com/j/acumen so old links will always work. This seems a minimal level of backwards compatibility (although I wonder if there is any specific protocol for how to implement this that will keep search engine mojo—but not a lot because the site gets most of its traffic from word of mouth between users).
The point of the article is that someone visiting the old URL should the old resource as opposed to a 404, an error, or some different content. If you can't keep the old URL the second best thing to do is a redirect. (EDIT: I guess being pedantic the point is to design the URLs so you don't need to change them later, but "get it perfect the first time" is kinda useless advice :-)
This is what 301 HTTP status (permanent redirect) should be for... [1] So it seems to me if you use 301 you should be good to go.
Also from a quick search it seems the recommended thing to do is remove the old URLs from your sitemap.
