I encountered this now, and have seen it quite frequently before: I would like to reject an edit, and the reason is something between "attempt to reply" and "clearly conflicts with author's intent". Usually (as in this case too) one cannot say there is no improvement, to the contrary, there is, sort of, too much improvement - to the point that it is almost a different question, maybe even better and more sensible; still, it is not clear whether the authors of the original questions would find it conflicting with their intent or not. All the same, I would prefer to reject the edit, and the reason I would name is that the edit introduces too radical changes.
I realize that I could explain myself in a comment after choosing "causes harm"; the problem is that I cannot also say it really causes harm, it is just that I find the edit too intrusive.
Also, since according to my experience the situations described are relatively frequent, and also in view of similar questions raised before, like Editing etiquette or Discreet editing, maybe it makes sense to add a corresponding reason to reject, although I am not sure which formulation to choose: something close to "too intrusive" but the latter I find sort of vague.
What do you think?