When you take admission in any IIT through a preparatory course, then you are not alloted any branch during the time of admission.

During first year you are not taught any engineering but only 11th and 12th science and maths and after 1 year you give exams.

Based on you performance in these exams, you are alloted a branch in your institute so getting CS only depends on how much hard work you can put in the first year.