See it doesn't matter how well you perform in the semester . Wether there is a vacant seat or not. ICT Mumbai doesn't allow branch chnage officially. Now by officially what i meant is that in ICT Mumbai you can attend any course lecture after first year, with the allowance of the faculty you can also give exam of other branch. So you can't change to chemical. So it would be better if you take branch of your choice.

All the best.