This post originates from this question, but is not about handling that question. I will summarize the transpired events for context.
The question is about a specific rule and at its core about whether that rule allows for a ridiculous scenario. It used an example which appeared to "break" that rule.
That original example had a flaw not related to that rule. In my answer I addressed that flaw and posed a different example which would work, before addressing the core issue.
Once OP saw that issue they changed their example to mine. This invalidated and necessitated revision to a lot of my answer.
To be clear, I am now happy with the state of the question and my answer and am not seeking to revert the changes made, however what our policy on acceptable changes to questions became apparent as unclear/unstated to me.
In particular this discussion was brought up, which states that answers should be edited to adapt to changes in questions. However this seems to differ to what I have experienced seeing. This particularly happens with homebrew questions, with a querent updating their material on the back of an answer. In this case policy has been to revert the changes and have querent post the new version as a new question as per our homebrew policy.
There are some previous metas relating to this (see this and this among others), however these, at least in part, address that the question were or should have been closed while edits happen to it. In these cases consensus seems to be that answers should adapt or be deleted as they were arguably posted too early. For the question spurring this post no closure happened, nor were necessary.
On the other hand complete changes to questions are not acceptable. Such changes should be posted as a new question instead.
In a similar vein to the spurring question a recent discussion decided a error correction should not be made to a question after an answer had addressed the mistake. This was asked by editors and when OP made those changes it was reverted.
The matter I would like answered is then, what kind of changes to a question is acceptable to make after an answer has been posted?