That is a very old issue.
This is someting I think about from time-to-time. It would definitely be an extremly useful features to have in some situations. (For example, you could exploar a lot of questions, do a search and replace of one word for another, and then re-import.)
It is not going to happen any time soon, but in Moodle 4.0 we should have a new improved question bank. Among other things, that will keep the version history of each question, and I think that makes this feature easier to implement in future. Import does not completely replace the original question, it just adds a newer version. So, perhaps after 4.0 is out, we can come back to this idea.