Most of it will depend on your hard work , your branch and the hard work done by your colleagues. For example – There are people who stood 5th or 6th in their class and were able to change their branch and sometimes only the top two get that opportunity.
If we go by the trends I have seen (for CSE only), generally mechanical students don’t want to change branch so it’s a bit easier for other mechies and most of the electrical students want to so it’s a bit tough for them. Because of the current batch strength of Chemical and Civil , I think, from these, only one student gets the opportunity to change his/her branch.