Assam Engineering College

Yes you'll get admission in AEC. As for the branch you might get to know that in counselling.

Yes, you can get admission in any of the above-mentioned colleges.

You might have a chance if you have any reservation criteria else give it a try. You have very great chance to get into this institute with this rank. Attend the counselling and choose your options appropriately. Try Shiksha college predictor for better analysis.

Please check for your eligibility and admission process followed to pursue B.E. in Mechanical Engineering at Assam Engineering College from the criteria mentioned at the link provided below.
https://www.shiksha.com/b-tech/course/b-e-in-mechanical-engineering-assam-engineering-college-guwahati-53449

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

As per my experience and as per government stats, Tezpur University is one of the most desired institution after IITs, NITs and IIITs because of its academics, study environment, student facilities and co-curricular activities due to which its the #1 Central University of India and among top 5 universities of India. Comparable to all things its placement that is not up to the mark as of now but will sure be awesome by 2 or 3 years.

Assam Engineering College admission in engineering degree courses is by Common Entrance Examination (CEE) conducted by the Directorate of Technical Education, Assam.