Hello there

According to me, if you are very passionate about doing M.Tech in Aerospace Engineering, you can do B.Tech in Mechanical Engineering first. Doing a B.Tech course in Mechanical is going to help you alot in knowing alot of concepts that you will be needing in Aerospace engineering. Although both the fields are not entirely or even nearly same, they compliment each other well, which means doing mechanical and then opting for aerospace is definitely going to be easier and profitable for you.  But then ofcourse, for the M.Tech course you need to clear GATE exam with good marks and then apply for aerospace, kudos if you can get into the best colleges like IITs.

Hope this helps. Good luck!