Another important point on modifying questions that already have answers is that it can lead to confusion by later readers. If I answer a question that says “Is it possible for X to happen?”, then the questioner decides the answer means the question is not what he meant and modifies to say “Is it possible for Z to happen when Y also happens?” a future user reading my answer will be understandably confused about why I’m bringing up X.
Even if you suddenly realize the question was silly, had an easy answer, is not what you really meant, etc., it is important not to modify it when questions have already been given. It’s not enough to comment saying “I’ve modified the question”, because you are now imposing on people and forcing them to go and read your new question and modify or remove their answer even if they are no longer interested in doing so. So it’s also a rather rude way to respond to help freely given. In Spanish, we say “Limosnero con garrote”, meaning “beggar with a cudgel”: you are asking for help, but making unreasonable demands on those who volunteer to help you.
In the cases at hand, you seem to have completely changed the question, thus making answers already given useless or wrong, and certainly confusing. That’s not really sociable behavior. This is different from simply making minor corrections (which is fine).
Added: One thing that you can do if you suddenly realized you left out an important condition that allows for “silly” counterexamples is to add to the question with a clear label indicating this is an addition, and perhaps mentioning people who pointed out trivial counterexamples which prompted the change. That way, people can see what led to the change and why the answers given addressed a prior version of the question. You know, like I’m doing here with this paragraph.