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auto re licensing
If this had a feature where one could set triggers for stuff, I'd use it to set up a trigger that re licenses all and each of my CC BY NC SA stuff to CC BY SA1 after each hits 50th year age.
The good part is I have more than 49 years to lobby for this cause.
One major problem with changing the license on a post after the fact is that it affects not just the original contribution, but also every change to that post made by others; [...]
When broadening the license of every revision of a post at once (strong, complex, composite). Not when broadening the license of a single revision (weak, simple, atomic).
This is a reason why many open-source projects, for example, are stuck with the license they originally picked; the people behind the project didn't bother to get a copyright transfer or otherwise a license from contributors that would allow them to change the license, so in order to change the license, they would need to rewrite all code where they can't get license change approval retroactively. For a large project, even identifying this code can be a mammoth task.
Nailed the golden nugget "the people behind the project didn't bother to get a copyright transfer". Thank you very much for contributing the key past evidence while concisely formulating the essence supporting the cause. Here is certainly asking precisely for the appropriate devices for giving "license change approvals" retroactively and for foreseeable futures.
In the scenario you describe, what happens to all the changes people contributed to that post over five decades while it was, and their changes were contributed, under CC-BY-NC-SA? [...]
Nothing immediate. Initial revision has been broadened to CC BY SA1, potentially triggering an action of another contributor who authored the subsequent revision contribution in that post, depending on the choices (if any at all) made by that subsequent author on the same or similar optional facility.
1: If that's a valid step, which am assuming it probably is and if isn't can probably be fixed by CC in the future.