Nope! The change of stream isn't allowed by the Calcutta University. If you wish to continue with the Computer Science then You have to take admission in the BSc Computer Science from the starting i.e., from the 1st semester. Now if you go for the BSc Physics then also you get quite a wide range of program skills such as Python in the paper of Mathematical Physics in your 1st semester and 3rd semester. In your 4th semester also, there are some complex python coding and data handling in Quantum Mechanics. In your semester 5 you will get quite a good knowledge of simulations using Python in Statistical Mechanics. This courses are quite good and as you learn them you can take admission in the MSc Data Science as well. So if you want you may not change your stream. But if you are not interested in Physics and want to shift, then you can obviously go for the course of BSc Computer Science but you have to take admission in the 1st year.

To know more about Physics as a career, visit the link below:

https://www.careers360.com/courses/physics-course

To know more about Computer Science as a career, visit the link below:

https://www.careers360.com/courses/computer-science-course

I hope this answer helps. All the very best for your future endeavors!