Biology

Yes,
Biology student can go for the BCA. There is no restriction.
You only need Logical understanding towards mathematical calculations and programming.
If you have interest in programming, go for it without hesitation.
All the best

Look for switching any different stream, it is purely based upon the determination and dedication of the student. It's many who switches between 12 and graduation just you need to know the basics accounts and little bit of management. In the course, you will learn a lot so don't worry.

Hi Jayananden, instead of the earnings figure out your interests to excel in your field. Your career won't grow if you don't enjoy. Know what you want, have a career plan and decide accordingly. Biology students popularly opt for one of the following
MBBS
B.Pharma
B.Sc Biology
B.Sc Agriculture
B.Sc Biotechnology
B.Sc Forensic Sciences
B.Sc Genetics
To name a few.
Research on the programs more and the career outcomes, see what interests you. You must have the aptitude and skillset too.

Eligibility criteria for seeking admission in B.Sc computer science is maths in 10+2 stage in majority of universities.

Yes, if you pass in +2 then you can change to arts.
(I wrote 'if' because if one fails, he will first have to complete +2 with arts stream, then he may proceed).
Good luck!

If you want to become a doctor, the best option is MBBS. Other then this, you may go after the following:
Radiology
Pharmacy
Physiotherapy
B.Sc (all subjects)
Applied medical science, etc.

Go for graduation from Delhi University. Admissions are based on class XII merit.