<p>To question # 1, yes you can do a B.S. in chemistry and then go to grad school and for something else (example: psychology). You will take the GRE exam, submit your application. You may have to take additional courses to fulfill the pre-requisite requirements of the upper level psychology courses. (I have a Bachelors in Business administration, got a Masters in Educational Psychology, now pursuing a PhD in psychology). </p>
<p>The example of going aeronautical engineering and going for an MBA is totally feasible because admission to most MBA programs are contingent upon having some work experience under your belt as well as you undergrad GPA and GMAT scores. As it would be very unlikely to go straight from undergrad to a MBA program, after working a few years, it is totally doable.</p>
<p>As far as getting a Bachelors in economics and going to medical school, that is also a feasible option as you can basically "major" in anything you want and still attend law/medical school. However, medical schools require you to have good scores on your MCAT, which means that you must be grounded in Chemistry, organic chemistry, biology and physics. Many students choose to go to school, major in what ever they want and then take a post-bac year in a pre-med program (example: JHU, Columbia, NYU, there are many post bac programs) then take the MCAT and apply to school.</p>
<p><a href="http://www.jhu.edu/postbac/%5B/url%5D">http://www.jhu.edu/postbac/</a></p>
<p>more extensive listing of post bac programs</p>