I am a PCM student. How can I change my stream? Would it be a good decision?

It depends on your attitude. If you can take it as a challenge, you can research your target. In future, you may have to perform more difficult tasks. So, work well and achieve what is intended.

Changing Science to Commerce is little bit hard. Subjects like Accounts and Economics will be little bit hard because they are new one but you can score best when you concentrate on these subjects. One plus point is this you are Science student and your calculation power will be very good and it wil