Hello Aspirant

The eligibility criteria for applying to an MCA course is that you should have Computer Science in your Bachelors or you should have done BCA. Since your Bachelor's is from the Arts stream, you are not eligible for the MCA course in any college or university. Your 12th subject will not matter in taking admission in a science field after you switch to the Arts/Humanities stream, the Master's level especially.

Hope this helps.