Hi Jay,

no, you cannot change streams like that.

if you want to do commerce you should take admission in 11th again in Commerce. 

You should remember that all competitions and entrance exams take 11th and 12th together and ask questions from both the classes. 

I would suggest you try hard and try passing 12th in science, it is not that difficult if you are ready to work hard.

After 12th you can choose to do any course you like. 

Hope this helps.