Can i change from BS Chemistry to BTech Chemical Engg. after 1st Year in IIT Kanpur ??

Just go with Yadav. I agree with his option and also I searched the internet, it's true.
All the best. Don't be sad, you'll definitely get it.
Also, strength of your parent department (BS Chemistry) shouldn't go below 55% of sanctioned strength. Last year, there was one case when student's application was rejected as the strength was left to 16 only.
So. Do consider all these parameters before taking a wise decision.
Hope this helps. All the best!
If you liked the answer, please feel free to upvote :))
Even if I'm wrong then you have to work very hard in first year of your college. You have to maintain cgpa of more than 9 if you really want to change your branch, that'll obviously require dedication and hard work. Moreover branch change from A to B also depends on no. of students in that branch. From branch should not have less than min% and to branch should not exceed max%.