Firstly if you want to join BTech courses after Diploma then you have to qualify lateral entry examination organized by the State government and there are no national level examination.

After Diploma many students change there area of study, so you can change it provided the college you want to study in must be a participant in the lateral entry exam.

To know more about Aeronautical Engineering visit the link below:

https://www.careers360.com/courses/aeronautical-engineering-course

To know more about the lateral entry examinations visit the link below:

https://engineering.careers360.com/articles/what-btech-lateral-entry-eligibility

I hope this answer helps.