Questions can be clarified, but new questions aren't clarifications and should be posted as separate, new questions.
The correct course of action is to revert to the last version that wasn't a different question. (Further edits may be appropriate to clean up, clarify, or whatever that revision still needs that didn't happen because of the new-question edits. This may be unnecessary if the question is closed or otherwise no longer of interest to the OP or anyone else.)
This is a topic that comes up semi-regularly, though I think this is the first meta question that has directly asked whether completely changing a question is OK. Related meta questions (the first two are the closest to this one, but I don't think exactly the same):
In general, there can't be a general rule. The most workable guideline though appears to be “if it's a new question, it goes in a new question post”. In other words, it relies on the “I know it when I see it” sense of the community.
There are of course muddy cases, but the general guideline is the place to start. One muddy case I've seen is when a badly-asked question was so badly asked that fixing it basically created a completely different question — in that case, it was better to abandon it (and its multiple answers) as unfixable so it could start over in a fresh, new question post.
On the other hand, sometimes the clarification could have been guessed from the beginning, making it a true clarification and not just a new question, despite many answers initially guessing the wrong way. In that case, these related metas are relevant: