Tim might jump in and provide the best answer, but I think that the technique now is to go to the questions bank first, duplicate the question, then edit it making changes.
Yes, I know exactly what you are talking about because I too remember the "Save as new" feature.
Yes, I know exactly what you are talking about your edits changing the original question. With moodle, you have to know that this is happening. Is this a "feature?" Well, I think that it is. I have lived with other LMS that do not have a centralized question database, and it is terrible trying to fix questions. From my own experience, moodle's quiz and question bank is the best. Again, one needs to understand its design to use it to their best advantage.
Having said this, I would also say that there is still room for many improvements. You can explore the suggestions in Moodle's Tracker system, and vote for those that you would like to see implemented. I can say that Tim, for example, and others are always seeking ideas for improvement.