To change your dept after 1st year the basic criteria is getting above or equal to 8.5 YGPA considering score of both 1st and 2nd semesters above or equal to 8.5 CGPA.

Then comes the questions like

1. if you are willing to change to a dept with better wbjee rank holders.
2. if the dept is in high demand.
3. the number of seats available in that dept.

Considering all of these, you will face the competition to change the department.

SO, its better you start early and try getting atleast 9.0 CGPA in each semester of 1st year.