If you are trying for CSE branch then you should expect a CGPA between 9.10-9.3. But this CGPA may vary as well. You should have a word had in the 1st semester so that there is no problem while changing to any branch. You should also analyze the competition level in the college as well so that you can predict the CGPA for this.
At IIEST Shibpur there are fixed number of seats which needs to be filled by the students of other branches like in the case of CS and IT they allow up to 4 students while in the case of ETC they take up to 2 students. The eligibility for changing the branch is done by your CGPA and applications are usually accepted after the third week of July i.e. right after the starting of third semester.
Lot of applications are there and it depends upon your CGPA which branch you will be allocated. If you have a good CGPA then you will have more chances for getting your preferred branch.