Hi

It is totally okay to change your stream from science to commerce after 12th.Bachelor of commerce offers understanding in Accounting,  Economics, Business Law, Taxation, Insurance and  Management.

For b.Com mostly the universities consider 12 standard scores. 

Eligibility 

Candidates from Science Stream- Minimum 60% aggregate AND 60% in Mathematics

Hi

First of all science student is not eligible for B.Com course. To be eligible for B. Com course student needs to be a commerce student. After 10 + 2 in commerce the student can apply for B. Com course into any desired college of his choice.

And second of all most of the colleges in India do not conduct entrance exam for B. Com course. They offer admission based on 12 std score