Tata Institute of Fundamental Research

No dear, this is not possible to do class 12th with Commerce, and then do B.Sc. So, you have to repeat class 12th with science stream, because both the streams are completely different from each other. And, B.Sc in physics is again a difficult one. But, if you are sure now to change your stream, then you must pursue class 12 with science again.
Yes, most importantly please think twice before taking any step, as it is about your whole life. Your single decision can change your entire life, so please make sure that you must be clear with what you are doing.

TIFR only offers post-graduate and doctorate degree courses only. There's no undergraduate courses offered by this university.
For more details, you can go on its official link.

Yes, Tata Institute of Fundamental Research, Mumbai (TIFR) does offer a course in B.Sc Physics, which is a full time UG degree of 3 years duration. The university is NAAC 'A+' Grade Accredited. Regards.

Hi Abhishek,, at present the integrated program in Physics is only for MSc and PhD. You can check the TIFR website for program details.

Admission Process
Admissions to all the programmes offered by TIFR (except science education) is based on a written test conducted by the institute. However, the institute also accepts GATE / NET / JEST scores for admissions to Physics.
Students who get shortlisted will be called for interviews which is generally held from Feb to April every year. Candidates will be intimated by email.