Hii,i am here to help you according to your query.Well,Any mechanical engineer after completing his B-tech can join for Aerospace stream for his post graduation if one has good GATE rank.But before changing the branch, you must be 100% sure about your future . Because in India there are very limited colleges such as IIT’s and NIT’s which offers you admission. So in future you may face lots of problem to join in Industry because your resume will completely change after your M-tech. Then only option remains for you after M-tech is to join for lectureship or PhD.Some of my friends have joined for lectureship in private engineering colleges after their M-tech in Aerospace. They did’t get any entry in Industry as trainee.

Hope,this information helpful for you.Goodluck.