In addition to what patrix mentioned - I like to keep in mind the purpose of a post:

- the site exists to draw the best answers, therefore questions exist to serve the answers 

So, add in some other facts:

- This question has many answers that are good
- This question is 3 years old now

At that point in a question's life cycle, I'm heavily of the mind that locking the question and having it serve the community is the best choice in most of the cases. If someone, even the OP wants to change things so that answers need to be changed, it's best to ask a new question at that point. 

The canonical question process is also something to think about where sometimes a post is locked and used to cover all the bases.

- http://meta.apple.stackexchange.com/questions/2418/where-is-the-list-of-canonical-questions-stored-for-ask-different

In that case, the follow on effect you mention is worth considering, so looking at the role of the posts sometimes helps to make editing decisions and/or reply when one person has a vision for the question that others (answerers, reviewers, moderators) see a different vision of the scope or the wording.