If you want to take admission in the course of BTech in Civil Engineering through the lateral entry examination then you can do it even after Diploma in Automobile Engineering. Switching of trades are allowed for lateral entry examination. Although I would advice you, if you wish to do so , then you must be very sure that you have to put in some extra efforts as the Automobile engineering is quite a different field compared to that of the Civil Engineering. SO you must be ready to work hard and if you have genuine interest in the subject then you will do great in it.

To know more about Civil Engineering as a career visit the link below:

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

I hope this answer helps. All the very best for your future endeavors!