hi,

the eligibility criteria for kvpy sx stream is, Candidates must studying in science stream in class 12. A candidate must secure a minimum of 80% marks for Gen category & 70% marks for SC/ST/PWD category in aggregate in Science & Mathematics subjects in High School Examination.

the eligibility criteria for SB stream is-  Candidates must be studying in their 1st year of Bachelor degree in any of the Basic Science related  courses. A candidate must have  a minimum of 60% in math and science subjects in class 12th board exams. For SC, ST and PWD, the minimum required marks is 50%. To avail the scholarship, the candidate must secure a minimum of 60% in the first year of his undergraduate course to take up the fellowship. For SC, ST and PWD, the minimum required marks is 50%.

so you are eligible for SB stream if you are currently enrolled in any bachelors science related course.