Hope you are doing good.

Preparatory course is an amazing way to prepare category students for the upcoming academic course. The only requirement of of levelling up from preparator re course to actual course is that you need to complete the the preparatory course and pass it.

if you have passed the proprietary course your seat which was reserved for you would be provided to you. So you have high chances that you would get naval architecture at IIT Madras.

If you have any more doubts you can ask them in the comment section.