Hari, both MD and MS are Postgraduate Medical courses whose minimum eligibility criteria is that you should be completing your MBBS with mandatory 1 year internship. Now let's say you appear for NEET PG and secure a seat in MD Medicine. You complete that course in 3 years. And within the next 3 years, NEET PG is going to be scrapped off in favour of NEXT. Which means, then the admission process will change. Though this happens, the basic eligibility criteria of MBBS being the mandatory qualification will remain. So you can study MS Surgery based on your MBBS qualification.