Rolling back a completely changed question

Question

Is Levenshtein slow in MySQL? was originally asked August 9, and received answers. A month later, the OP completely changed the question, invalidating the existing answers. I rolled it back, and left a comment to say that a new question should be opened, but the OP changed it back saying "There is no rule about it, so you can't edit my post."

There is also a bounty on this question, making things more interesting.

What are the guidelines on completely changing a question or rolling back changed questions? Was I out-of-line rolling back the question?

I have no desire to get into an edit war, so I'm not going to touch the question again. If a moderator wants to get involved, great.

Answer 1

If the question was completely changed, and the already existing answers were not anymore valid after the edit, then you were right to rollback the question. That is even more true if the question was a valid question before the edit.

In the specific case, the question passed from "Is Levenshtein slow in MySQL?" to "What classes should I use in my project? What else should I know about OOP?" which means the topic is radically changed. If somebody had read the new question, s/he would have wondered how somebody could have answered about the Levenshtein distance when the question is about using OOP in a project.

Re-using a question to ask a different question (which is what also happens when users change a question they asked instead of asking a follow-up question) is wrong because the answers are not thought to be useful just to who asked the question, but also to future readers. This means I cannot delete a question for which I got an answer, just for the fact I now know the answer; it also means I cannot re-use a question to ask a completely different question, for the same reason.
As a matter of fact, if the question is only useful to who asked it, then it is a candidate for being closed as too localised.