How to relicense content with unknown license as CC-BY-SA?
<p>I have recently became an owner of a creepypasta story website based on Mediawiki. This website seems to have no explicit content license, and the only legally binding text shown when creating or editing pages is following:</p>
<blockquote>
<p>Please note that all contributions to <code>{{SITENAME}}</code> may be edited, altered, or removed by other contributors. If you don't want your material to be edited mercilessly, then don't submit it here.</p>
<p>You are also promising us that you wrote this yourself, or copied it from the resource that allows free redistribution and modification of their content.</p>
</blockquote>
<p>Which is very similar to old <a href="https://en.wikipedia.org/w/index.php?title=MediaWiki:Copyrightwarning2&amp;oldid=56654122" rel="nofollow noreferrer">Mediawiki copyright warning</a>. The website also does not have any legal entity associated — domain and hosting are owned by non-entrepreneur private person, and website itself does not make any profit — there are no ads, no donations etc.</p>
<p>I would like to make all aforementioned content explicitly (re)licensed under CC-BY-SA 4.0 or CC-BY-NC-SA 4.0. Can I just claim that all website content is now under CC-BY-SA, or do I have to perform any additional actions?</p>
<p>How should I deal with orphaned works (e.g. creepypasta obtained from unknown sources, or written by authors that have left no contact information)? Does the aforementioned notice give me the right to threat all that content as compatible with CC-BY-SA? Who should I attribute as an author then?</p>
<p><em>Disclaimer: I am not looking for real, qualified legal advice here, so no strings attached.</em></p>

<p>You can do whatever you like with posts made after you change the rules - you have to leave the previous stuff alone.</p>

<p>The contributors' have accepted the terms of the licence:</p>

<ul>
<li>They own the copyright or have permission from the copyright holder to post it (the promise)</li>
<li>They agree that it can be edited altered or removed</li>
</ul>

<p>CC-BY-SA  allows people to copy the stuff off the website and republish it - this is <strong>way</strong> outside what the contributors agreed to. These people have given permission for their work to be altered but <strong>not</strong> copied.</p>

copyright licensing