I would say this is basically the answerer version of "moving the goal posts", a behavior discouraged by OPs. For those who don't know what that means, it refers to someone asking a question, getting an answer to it (we'll suppose it is pretty good: it gets upvotes, seems to answer the original question, etc.), and then editing their question in such a way as to make that answer essentially meaningless. Clarifying the question, such as improving the language or adding further requested details, is fine, but a wholesale alteration of the fundamental question itself is considered bad form. An exception is if there are currently no decent answer attempts posted. Otherwise, for posters who feel they need to completely alter their question to get "what they really wanted to know", it is generally encouraged that they post a new question entirely.
The short answer to your question is then contained in the original comments: alterations for the sake of basic maintenance is okay, but "wrenching" an old answer or otherwise fundamentally invalidating the community's votes and answer acceptance is bad.
This is, after all, a Q&A set of sites generally meant not to satisfy the posters and the responders, but of anyone who happens to think of much the same question and wanders in here off of a google search or whatever. While an OP may decide he botched his question, or an answerer may change his opinions, if it was a well-formed and answerable question, or a well-formed and positively upvoted answer, then it should remain: somebody else may have the same question and find the answers helpful.
I will note that if your answer is not the accepted answer, then you may simply delete your answer at any time (as long as you are a registered user), regardless of how many upvotes it may or may not have. You can then post a new answer, or not, as you desire. If your answer is the accepted answer, then you cannot simply delete it, and your recourse is to post a new answer if you wish to make a significantly different argument. You can post as many answers as you desire to any given question, as long as they are earnest attempts to answer the question(s) at hand, and are in some way meaningfully distinguishable from each other. A question on the Mathematics SE may have an answer which could be derived in a dozen valid and different ways, and it is generally acceptable for a single person to post a separate answer for each one.
Exactly where the line is between "improving" and "fundamentally altering" may be hard to tell at times. I would suggest that if you are unclear on which one you are doing, then go ahead and play it safe and make an entirely new answer. It's easier to placate a community that thinks you should have just merged them into one answer than a community that thinks you're being deceitful.
Similar reasoning would apply to your last question. If you decide the new information does not lead to a significantly different answer, then it seems to be accepted form to add this into the original answer with some sort of clear indication that additional material was added rather late. I would not be opposed to it being given its own answer, either. For answers that seem to have been invalidated by new information, posting a new answer and adding a disclaimer to the original post, such as "This answer was made before Chapter 207 came out, and so may no longer be completely accurate or relevant", would be good. The option to delete is also available to an unaccepted answer, if you feel that is best.
