Hi Nasir,

You cannot simply shift from one course to another without having given its entrance at Jamia. B.com and BBA are two distinct courses with desperate entrance tests for each and not like a B.tech where you could switch streams. If you have given both the entrances, you surely can choose one if you've got a meritorious score in both. However one entrance's merit cannot be used to claim admission for another course. 

All the best!