I have some serious concerns with this, especially after the last round of licensing fiascos around the MIT license.
I want to be clear: I like the Creative Commons licenses for text. I think that CC-BY-SA is appropriate for content contributed to Stack Exchange. I think that CC-BY-SA 4.0 is a fantastic license and I use it myself.
However, you have not obtained my permission to change the license of the content that I have contributed. The Terms of Service did not require me to grant that permission as part of using the service (and even if you added it, you would need to handle the case where someone decides to stop using the service but does not grant permission for existing content) and I have not received a request for permission to update the license of my content.
My biggest concern here, like it was in the last rounds of licensing discussions, is a fundamental misunderstanding of the rules and laws around licensing. This does not give me confidence.
I'd like to make my concerns more explicit.
Before September 5, the Stack Exchange Network Terms of Service was an agreement between the users (myself) and SE that I agreed to by using the service.
One of the agreements in the pre-September 5th TOS was an agreement that I license "and and all content" that I provide to the public Network be "perpetually and irrevocably licensed to Stack Overflow on a worldwide, royalty-free, non-exclusive basis pursuant to Creative Commons licensing terms". Per the TOS, this is a "perpetual and irrevocable right and license to access, use, process, copy, distribute, export, display and to commercially exploit such Subscriber Content".
I would point out that I do not grant anyone the right to change the license of my contributions without permission in the Terms of Service, so we must consult the Creative Commons licensing terms that I am granting.
The Creative Commons licensing terms linked to was CC-BY-SA 3.0. The original answer here makes it clear that at some date, CC-BY-SA 3.0 was linked to. During an update, the wrong version (4.0) was linked to. This appears to be unintentional and wasn't communicated in change notes to users to give adequate information to make an informed decision on accepting the new Terms of Service.
For the sake of argument, we can say that this change to the ToS is fine - SE can arbitrarily require new contributions to be licensed under whatever they ask. It is up to the users to understand that. The issue is with past contributions.
To be clear on definitions, an individual contribution to a site on the SE network is a Work (see 1.h). A set of questions, answers, and comments that is made available is a Collection (see 1.b). I'd point out that SE content is not an Adaptation (1.a).
When I submit a post to an SE site, I granted SE a perpetual and irrevocable right and license to that post under CC-BY-SA 3.0. They then use this license to make my post available to others but also to form a Collection with other posts also submitted under CC-BY-SA 3.0. Prior to September 5, both the individual Works by various people plus the Collection were CC-BY-SA 3.0.
4.a in CC-BY-SA 3.0 states that "You may Distribute or Publicly Perform the Work only under the terms of this License". In this case, "You" is someone who received the Work under the license. This is a prohibition on relicensing Works. This section also addresses Collections - "This Section 4(a) applies to the Work as incorporated in a Collection, but this does not require the Collection apart from the Work itself to be made subject to the terms of this License."
4.c in CC-BY-SA addresses distribution of the Work in a Collection. It requires that all copyright notices remain intact. Prior to September 5, the Collections were also distributed under CC-BY-SA 3.0.
8.a makes this even more clear: "Each time You Distribute or Publicly Perform the Work or a Collection, the Licensor offers to the recipient a license to the Work on the same terms and conditions as the license granted to You under this License." Again, "You" refers to SE distributing individuals Works in a Collection.
The new Terms of Service require new contributions to be licensed under CC-BY-SA 4.0. This is OK and acceptable - totally fine for SE to do at any point in time. However, due to the fact that permission to relicense was not granted, the license of existing works cannot be unilaterally changed.
Given that the ToS did not grant permission to relicense works and CC-BY-SA 3.0 explicitly prohibits relicensing works and the CC guidelines for migrating Works were not followed, I'd like an actual answer from SE identifying (with quotes) exactly what sections of the Terms of Service or CC-BY-SA 3.0 are believed to grant them the rights to relicense my contributions.
I'd note that I am not objecting to the collection of works being licensed under CC-BY-SA 4.0. I'm also not objecting to new content being licensed to SE under 4.0.
I simply want SE to demonstrate a basic understanding of copyright and licensing - right now, I'm very quickly losing trust in SE to appropriately handle my contributions.