For as long as this site has been operational (or at least as well as I can remember) we have strongly discouraged "moving goalposts" questions. Generally if no answers are yet posted clarifications, revisions, and even outright changes are OK (as it sometimes takes a bit of a dialog to develop a proper question) but once answers start coming in the fundamental nature of the question should not change. This does not mean that the interpretation of the question cannot change; sometimes people just misunderstand the question which lets the OP know it may need further clarification. (e.g. Bob Hanlon's answer to this is wholly unrelated to the question I intended to ask; I'm still trying to think of a way to ask it more clearly however.)
Unwritten(?) moderator policy has been as J.M. already commented:
I'd roll back the question to the last relevant version and then "subtly" nudge the user to ask a new question, since it is quite different from what he was originally asking.
Next time you spot this happening in a timely fashion (before new answers start appearing) you should flag the post for moderator attention, and/or perform the roll-back yourself, with a polite explanation. If the OP contravenes your action do not repeat it; leave it to the moderators to handle.
If I am the only person to have answered I may allow a major change to the question but it is always appreciated if someone asks first.
A gray area in this policy that could use some attention is extensions to existing questions. Sometimes these clearly seem to be clarifications of the author's original intent or problem, but other times they are simply follow-up questions. I wonder which serves the community better in the long term: splitting off the follow-up to a separate Q&A which may make related information harder to find, or allowing the extension which makes existing answers incomplete?