I agree with Tussy's post- Medicine and Nursing have completely different scopes of practice and knowledge. If I were in your situation, I'd try and figure out which one suits you better as early as possible. Have you job-shadowed with both occupations and compared? Even one weekend of your time could make a huge difference in thinking about all of this.
There are pros and cons to majoring in nursing. On the plus side, RN is a a solid career path with good job opportunities, relatively high/stable income potential, and is a highly respected profession. It does allow for a broad training in science/patient care that is suitable preparation for medicine although not as detail-oriented as a major in a typical science degree.
To be honest, as an interviewer I'd look down on the fact that you used up an RN spot to deliberately springboard into something else (if that was your intention all along). Committees may give you extra attention (not always in a good way) and question why you are leaving nursing (especially with a nursing shortage in most areas, be prepared to answer that at interviews). However, there are many RNs that apply to medical school every year and many do get admission, but you'll have to research the stats and see where you'd have the best chance with such a background.