B.Tech

Yes, you are eligible to pursue aeronautical engineering course after the completion of your mechanical engineering Diploma course. Candidates who have completed 12th standard with non-medical science stream or those who have completed three years Diploma course in Mechanical/Aeronautical Engineering are eligible to pursue aeronautical engineering course. To get admission in aeronautical engineering course after the completion of your three years Diploma course, you need to appear and qualify lateral entry examinations like LEET/ECET/JELET, etc. By qualifying the entrance examination based upon your examination rank, you will be allotted a seat in a particular college and you can get admission in the second year of four year B.Tech course. But I would suggest you continue with mechanical engineering because it has a huge scope in near future and you can also go in the aeronautical field after that.
Best of luck.