For CSE, you will need a CGPA of above 9. Naturally, the required CGPA varies each year, depending on competition, seat availability, etc. CSE is the most sought-after, so the CGPA requirement usually stays high. There are also other additional rules regarding branch changes. IIT Madras allows a maximum change of 10% in both branches.
There have been some changes in the guidelines for branch change at IIT Madras in the last few years. If a student applies for the process of branch change then the marks of two subjects, namely physics and mathematics would be taken into consideration, and as the subjects of all branches are the same so the rule goes the same for all the branches.
The second criteria are CGPA. For example, let’s say there are two students namely X and Y. If X attains an S in both Physics and Mathematics with a CGPA of 9.3 and Y attains S in Mathematics and A in Physics with a CGPA of 9.8, then X would be given the chance to change the branch.
Therefore, it's advisable to attain S in both physics and mathematics if you wish to change the branch with at least a CGPA of 9.4 or above.