IIT Mandi - Indian Institute of Technology Answered Questions

College change or branch change after the first year is completely dependent on college norms, some college allow it and some won't plus you need to have good scores in the first year to apply for the same. For further details contact the college authorities.

First of all congratulations! I will recommend you to go with CSE from IIT Mandi, but in that case only if you have a keen interest in computer science and engineering. Placement wise it's very good also you will get very good exposure. IIT Mandi has very good semester exchange program too, you have a very good option there to go abroad and explore further. Else if you wish to go with core opt for the other options which are left.

Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including works like roads, bridges, canals, dams, and buildings. Civil engineers design, build, supervise, operate, and maintain construction projects and systems in the public and private sector, including roads, buildings, airports, tunnels, dams, bridges, and systems for water supply and sewage treatment. Many civil engineers work in design, construction, research, and education. Civil engineering is an age old profession and with Government of India focus on infrastructure scope and opportunities for Civil Engineer will be very high for next few years.
You are advised to appear in JEE Mains and JEE Advanced to seek admission in IIT Mandi for B.Tech civil engineering.

I can clear your doubts regarding IITJ and for IIT Mandi, I would suggest to you get connected with some person there. And one more thing asking people the choice will show their perspective, hence please ask for specific details and collect answers and then compare for yourself!
IITJ is a great place and if you choose to come, then you will enjoy your four years here as I did. Placements here are great for CSE, wherein almost 100% of students get placed in companies of desire. Some even end up with high-end packages in multinational giants such as Google, Microsoft etc. Currently, IITJ has a temporary campus, but as per the latest info, IITJ will shift to a permanent campus this year in Diwali 2016, with all the wanted and expected facilities! But, even after being on a temporary campus, you have all the needs you require or you would expect from an IIT. I would suggest looking upon the following sites where most of your questions will be answered: iitj.ac.in , cs.iitj.ac.in , spc.iitj.ac.in
Feel free to ask for any of your doubts!

Check the links below for rank and college prediction :
https://www.resonance.ac.in/jee-main-answer-key-solutions/jee-main-2016-rank-predictor.php
https://www.shiksha.com/jee-mains-college-predictor
Stream should always be given preference (doesn't mean you join any college) because that is what you love and want to make in career in so choose the desired stream and then opt for college accordingly