Let's all hope w3.org doesn't forget to renew their domain. Because if they do, all these links to "Cool URIs don't change" will suddenly become extremely ironic. We're safe at least up to 2029...
If URIs aren't permitted to change, they require inalterable time and/or content elements to them. Probably ownership/authorship as well.
Because otherwise, this becomes an increasingly futile (and frustrating) Quixotic quest.