Can I opt for aerospace engineering after completing BTech in civil engineering

You said that IIT Bangalore has a quota for the civil engineers for doing Aerospace Engineering. So, one can definitely try for that and if one gets admission then doing it from IIT makes sense. Anyway, I am not aware of any institute in India which has a quota for civil engineering graduates in Aerospace Engineering.

On the other hand, there is some management quota in many engineering colleges which a person can get without appearing for JEE or any such exam. That is called direct induction in engineering course. I think you can find that path in the institute of your choice. It is also to be noted that you would be incurring another 4 years period in this pursuit which is a valuable time at this juncture in your life and you could settle for some alternative career by that time in your core proficiency. I think you have to rethink and take a conscious decision in the matter.

After completing your Civil Engineering course, you can do post-graduation in aerospace engineering. The following institutes are offering this course for civil engineers.
1. IISc, Bangalore
2. IIST,
3. IIT-Bangalore,
4. IIT -Kanpur,
5. IIT- Karghapur,
6. IIT- Chennai.
7..SRM University and many other institutes are offering some seats in aerospace engineering for other branch graduate engineers.

The GATE score is the criteria for admission. So concentrate on GATE score.
You will get all the details from the web site of the institute.

People have more money does not mind about job/business lest they put some shop without minding the business/profit/sales.
Similar to that if you have money and not minding about job or earning you can study further and further as you wish irrelevant courses.
Otherwise you would planned your course earlier itself . If civil engineering is not interested one to you, you would opt for your course as per your interest. In case you were not getting admission in one year you would have wait for next year instead of wasting years in uninterested course.

Your idea to take up admission in Aerospace- engineering after B. Tech in the Civil engineering does not appear to be a sound decision because of the following reasons -
1) While considering the time span for completion of the course in B.Tech in Aerospace- engineering may take as many as three in case you are admitted at the lateral level in the second year. Hence the total time span under such cases would be seven years.
( Four year for B.Tech in Civil Engineering and three year for Aerospace engineering as a result of your lateral entry in the second year.)
2) Loosing three years would amount to a great loss in terms of your service career.
3) Money invested by way of acquiring these two degrees cannot be compensated in the initial years of your service - life.
What I say that you need to modify your decision in such a manner that you could save your substantial time and you may get a better plateform in terms of your service career if you take up the following routs -
1) Appear for the GATE examination and make an aim to secure beyond 98 percentile so as to secure admission in M.Tech in Aerospace - Enginnering in IIT's , IISC, Bengaluru etc.
2) Aeorospace - engineering contains two main areas such as Air Craft Frame Structure design and Aerodynamics- analysis and you have had enough exposures in these areas while doing your B.Tech in Civil Engineering. This could help you in your conceptual clarity in your M.Tech course in Aerospace Enginnering.
3) There are a lot of demands of this post graduate qualification in ISRO, DRDO, HAL etc.
4) You can qualify GRE so as to take up assignments in the foreign jobs.
Yo can look for the following institutes for your admission in M.Tech -
1) IIT Kharagpur
2) IIT Mumbai
3) IIT Madras
4) IISC Bengaluru
5) SRM University etc.

'Aerospace Engineering' is an engineering branch that deals in designing, construction, development, testing, production and maintenance of aircraft along with related set-ups like missiles, rocket-propulsion, space-shuttles, etc. It was known as 'Aeronautical engineering' but the expansion in the technology that integrates the function of the aerospace vehicle, the tag was restored by "Aerospace Engineering".

As you have completed your Engineering in Civil, there are very few institutes that offer M.Tech Programmes in Aerospace Engineering for other streams. Some of the institutes, their eligibility criteria are given below for your review:

Eligibility Criteria:
i) Candidate must have 60% marks for General and 55% marks for SC/ST candidates in their B.E. / B.Tech. or equivalent degree course.
ii) Candidate should have a valid GATE score in their disciplines.
iii) Candidates having work experience of 2 or more years in the pertinent field are exempted from the GATE score.

Courses available for Civil candidates are:
i) Aerodynamics(AE1) - Bachelor's degree in Mechanical Engg./ Civil Engg.
ii) Aerospace Structure (AE4) - Bachelor's degree in Mechanical Engg./ Civil Engg.

Institutes or colleges:
i) Indian Institute of Technology (IIT), Bombay - Department of Aerospace Engineering, Powai, Mumbai-400 076.
Website: https://www.aero.iitb.ac.in/home/postgraduate-program
Email: office@aero.iitb.ac.in
Phone: +91-(022) 2576 7101 / 7102

ii) Indian Institute Of Space Science and Technology (IIST) -Valiamala P.O., Thiruvananthapuram - 695 547, Kerala.
Website: https://www.iist.ac.in/
Email: admissions@iist.ac.in ; academics@iist.ac.in
Phone: +91 (0471) 2568477 / 2568478

iii) Indian Institute of Technology (IIT), Kanpur - Kalyanpur, Kanpur -208 016
Website: https://iitk.ac.in/aero/
Email: infocell@iitk.ac.in; aracad@iitk.ac.in; doaa@iitk.ac.in
Phone: 0512-259 0151 / 392-7200, 7210 / 0512-259 7669

iv) Hindustan Institute of Technology and Science - 1, Rajiv Gandhi Salai (OMR), Padur, (Via) Kelambakkam, Chennai - 603 103
Website: http://www.hindustanuniv.ac.in/
Email: info@hindustanuniv.ac.in
Phone: 1800 425 44 38 (Toll-Free) / +91 (044)27474262, 27474395

v) Indian Institute of Technology (IIT), Kharagpur - Kharagpur - 721302
Website: http://www.iitkgp.ac.in/
Email: registrar@hijli ; chrgate@adm ; vchrgate1@adm
Phone: +91-3222-255221

vi) University of Petroleum and Energy Studies(UPES) - P.O. Bidholi Via-Prem Nagar, Dehradun-248007
Website: https://www.upes.ac.in/
Email: enrollments@upes.ac.in
Phone: 1800-102-8737(Toll-Free)/ +91-8171979021/ 22/ 23, +91-7060111775

If you are planning to join another four year engineering degree programme it will not be a wise decision as you will have to spent another four years. Further you will have to spent a huge course fee also. Overall double engineering degree courses I.e. bachelor degree will not give you much benefits.

But if you are thinking to do a post graduate course or masters in aerospace engineering. It will be worthy and advantageous to you.

Indian Institute of Technology, Bombay Department of Aerospace Engineering offers M.Tech in Aerospace Engineering. In this course candidates who have engineering degree other than aerospace engineering are also eligible for admission. But the condition is that they must have valid GATE score in specific discipline. If the candidate have work experience of two years in relevant area, in that case they are exempted from GATE score requirement.

Besides, IIT M several other institutes are which offers M.E/M.T.ech in Aerospace Engineering. Some of them are as listed below:
1. Indian Institute of Technology, Kharagpur, West Bengal
2. Indian Institute of Technology, Madras, Tamil Nadu
3. Indian Institute of Technology, Kanpur, Uttar Pradesh
4. Indian Institute of Technology, Guwahati, Assam
5. Indian Institute of Space Science & Technology, Thiruananthapuram, Kerala
6. Indian Institute of Technology, Jammu
7. Institute of Aeronautical Engineering, Hyderabad, Telangana
8. Jawaharlal Nehru Technological University, Hyderabad, Telangana
9. Indian Institute of Science, Bengaluru, Karnataka
10. Jaipur Engineering College, Jaipur
11. Chandigarh University, Chandigarh
12. Jain University, Bengaluru, Karnataka
13. Birla Institute of Technology, BIT-Mesra, Ranchi, Jharkhand
14. Bursa Institute of Technology BIT- Since I, Than had, Jharkhand
15. Manipal institute of Technology, Udupi, Karnataka
16. Amity University, Noida, Uttar Pradesh
17. KL university, Guntur, Andhra Pradesh
18. College of Engineering, Anna University, Chennai, Tamil Nadu
19. Punjab Engineering College, Chandigarh
20. Amrita Vishwa Vidyapeetham, Coimbatore Tamil Nadu

After doing ME or M.Tech. in Aerospace Engineering you will be able to get into government or private orgNization. Now-a-days many private industries provide employment opportunities to Aerospace engineers. Some of the organisations that hire Aerospace Engineers in various capacities are given below:
1. Indian Space Research Organisations (ISRO)
2. Defence Research And Development Organisation (DRDO)
3. Hindustan Aeronautical Limited (HAL)
4. National Aerospace Laboratories (NAL)

Private companies: Boeing, Airbus, GE and Goodrich hire Aerospace Engineers for their aircraft related projects.