In general, any edit that changes the question should be rolled back, but for a question without answers, sometimes I think the OP can be given a little leeway as long as they aren't trying to get around a question ban and/or completely changing the question.
However, any edit to a question, other than clarifying the original question, that invalidates any answer should be rolled back and a comment should be left indicating what they did wrong. Sometimes it is possible to edit the question in such a way as to integrate the new info added by the OP without invaliding answers and the editor should try to make an effort to do that.
It's not the same as your example, but I think the resolution is still the same, but in this question, trouble adding calculated variables c#, I did exactly that. The OP, in an attempt to provide more code requested, removed what was wrong with his code originally and invalidated virtually every answer. So I rolled back the question, and then tried to incorporate the new code in such a way that kept up with the original spirit of the question, did not invalidate answers, and still helped the OP get the answers he needed. Based on his response to my comment when I explained what I did, he seemed satisified, so I tried to use this as a guide for doing this in the future.
I think most of the time, the OP doesn't know and usually is accommodating to the site's policies, so you shouldn't meet much resistance.