Is Stack Exchange allowed to unilaterally change previously published content license?
<p>The original thread can be found here: <a href="https://meta.stackexchange.com/questions/333089/stack-exchange-and-stack-overflow-are-moving-to-cc-by-sa-4-0">Stack Exchange and Stack Overflow are moving to CC BY-SA 4.0</a></p>

<p>As far as I understand, Stack Exchange can move on to CC BY-SA 4.0 for content published in the future, but they are not allowed to unilaterally change the license of previous publications. Several threads have been created requesting answers regarding this issue from Stack Exchange, however, <a href="https://meta.stackexchange.com/a/333912/619812">the only answer from moderator</a> did not give any meaningful explanation. </p>

<p>Since this is a legal issue, I would like to ask of Law Stack Exchange community to give expert opinion on this matter.</p>

<p><strong>Can Stack Exchange change the license for previously published content without the consent of users?</strong></p>

<p>What SE can do is controlled primarily by the <a href="https://stackoverflow.com/legal/terms-of-service">Terms of Service</a>. What most matters is the section on Subscriber Content, which says:</p>

<blockquote>
  <p>You agree that any and all content, including without limitation any
  and all text, graphics, logos, tools, photographs, images,
  illustrations, software or source code, audio and video, animations,
  and product feedback (collectively, “Content”) that you provide to the
  public Network (collectively, “Subscriber Content”), is perpetually
  and irrevocably licensed to Stack Overflow on a worldwide,
  royalty-free, non-exclusive basis pursuant to Creative Commons
  licensing terms (CC-BY-SA), and you grant Stack Overflow the perpetual
  and irrevocable right and license to access, use, process, copy,
  distribute, export, display and to commercially exploit such
  Subscriber Content, even if such Subscriber Content has been
  contributed and subsequently removed by you...</p>
  
  <p>This means that you cannot revoke permission for Stack Overflow to
  publish, distribute, store and use such content and to allow others to
  have derivative rights to publish, distribute, store and use such
  content. The CC-BY-SA Creative Commons license terms are explained in
  further detail by Creative Commons, but you should be aware that all
  Public Content you contribute is available for public copy and
  redistribution, and all such Public Content must have appropriate
  attribution.</p>
</blockquote>

<p>This part has not changed: the purported license is still "CC-BY-SA", and the TOS does not explicitly specify a version. What apparently has changed in the relevant section is one "helpful information" link, which now points to <a href="https://creativecommons.org/licenses/by-sa/4.0/" rel="noreferrer">https://creativecommons.org/licenses/by-sa/4.0/</a>. So the interesting question arises whether that would constitute an unpermitted post-hoc change in the terms by which SE has license to my older stuff. This matter came up in a reviled <a href="https://meta.stackexchange.com/questions/300465/does-the-license-given-to-stackexchange-forbid-the-additional-terms-terminate/300479">Meta question</a>; as <a href="https://meta.stackexchange.com/a/300479/334343">I pointed out</a>, the TOS also included a merger clause that</p>

<blockquote>
  <p>This Agreement (including the Privacy Policy), as modified from time
  to time, constitutes the entire agreement between You, the Network and
  Stack Exchange with respect to the subject matter hereof. This
  Agreement replaces all prior or contemporaneous understandings or
  agreements, written or oral, regarding the subject matter hereof.</p>
</blockquote>

<p>Because of that, the TOS is self-contained and stuff found on other web pages are not part of the agreement. This in itself is a bit of a problem because you can't both say "we're not bound by stuff outside of this page" and say "the specific terms of the license are outside this page".</p>

<p>That particular clause is gone, but there is an analog in the current TOS:</p>

<blockquote>
  <p>These Public Network Terms represent the entire agreement between you
  and Stack Overflow and supersede all prior or contemporaneous oral or
  written communications, proposals, and representations with respect to
  the public Network or Services or Products contemplated hereunder.</p>
</blockquote>

<p>Furthermore, the TOS contains the following "we can change it" clause:</p>

<blockquote>
  <p>Stack Overflow reserves the right, in its sole discretion, to modify
  or replace these Public Network Terms, as our business evolves over
  time and to better provide Services and Products to the Stack Overflow
  community, or to change, suspend, or discontinue the public Network
  and/or any Services or Products at any time by posting a notice on the
  public Network or by sending you notice via e-mail or by another
  appropriate means of electronic communication.</p>
</blockquote>

<p>I assume but do not know for a fact that a similar clause existed in prior versions of the TOS. So I conclude that the change is legal.</p>

<h2>Yes</h2>
<p>The relationship between Stack Exchange (SE) and it’s users is governed by the <a href="https://meta.stackoverflow.com/legal/terms-of-service/public#licensing">ToS</a> - a contract subject to all the normal contract law applicable in <a href="/questions/tagged/new-york" class="post-tag" title="show questions tagged &#39;new-york&#39;" rel="tag">new-york</a> due to the choice of law clause it contains. Common law (unlike civil law) treats licences no differently from any other contract (See <a href="https://law.stackexchange.com/q/6263/344">What is a contract and what is required for them to be valid?</a>). Note that this is only the latest version, versions back to 2010 can be found <a href="https://web.archive.org/web/2015*/https://meta.stackoverflow.com/legal/terms-of-service" rel="nofollow noreferrer">here</a>.</p>
<p>Changing a contract requires one of two things:</p>
<ol>
<li>Agreement of all parties with all of the requirements of a new <a href="https://law.stackexchange.com/q/6263/344">contract</a>.</li>
<li>A provision in the contract allowing the change, commonly referred to and sometimes specifically identified as a variation clause.</li>
</ol>
<p>The discussion on the linked question is focusing on 1. - SE’s legal authority comes from 2.</p>
<p>Specifically, the Creative Commons licence is only <em>part</em> of the contract between SE and it’s users: it cannot be read in isolation. There is nothing unlawful about incorporating documents into a contract by reference: indeed if you think of complex contracts like those in building and construction there is no practical way not to do it.</p>
<p>The ToS give SE the right to change the ToS unilaterally - in the current ToS its in clause 10.d and in the 2010 ToS it’s in Clause 13.</p>
<p>This allows them to change <em>anything</em> including the copyright licence.</p>
<p>There is no conflict with the CC licences, from the <a href="https://wiki.creativecommons.org/wiki/4.0_upgrade_guidelines" rel="nofollow noreferrer">upgrade guidelines</a>:</p>
<blockquote>
<p><strong>Existing content:</strong></p>
<p>…</p>
<ul>
<li>If the contributors, then need permission to relicense. Without permission (<strong>via terms of use or otherwise</strong>), then that content remains under prior version.</li>
</ul>
</blockquote>
<p><strong>Everyone who has ever posted agreed that the licence (and everything else) could be changed when the ticked “I agree to the ToS” when they created their account.</strong></p>
<p><strong>Limits</strong></p>
<p>There is nothing wrong with allowing unilateral changes to the contract, however, in the US the exercise of a contractural power must be done in good faith, that is, with due consideration for the negative impact on the other party.</p>

licensing