Hello,

I think you know about the syllabus pattern of 11th and 12th standard.  The portions of 12th standard are always in continuation with 11th standard portions.  So, it is advisable not to switch streams half way along the higher secondary.  If you still wish to switch streams after 11th standard, you can do the same.  You can approach you Headmaster/Headmistress with a letter stating proper explanation to switch streams.

Also, you should know that switching streams is a long and tedious process and so none of the institutions recommend switching them.  If there are available seats, then the HM may insist the admission cell to allot the seats of your choice in your chosen stream.