The common solution when a website changes its url scheme, is to setup http redirects. So the work stays in the hands of the website owner to maintain their list of redirects.
This solution currently suffices, because search engines then update their index entries with the new url they got from the redirection, but I can see this solution not being prefect in the long term, since the canonical link entries in the blockchain can't get updated after a certain time. So either the website owner forever has to maintain an ever growing redirection table, or you just let the links break.
RE: Steem improvement proposal : Canonical links.