Originally Posted by r.i.p

You will get three opportunities to revise your choice....
1. before the psychological
2. during the psychological written test
3. before the interview

Wait for your official letter declaring that you have cleared the CSS 2010 written exam. There will be a form alongwith it which also contains revision of preferences of occupational groups.

During psychological (written part) on which ur first 3 preferences are engraved. They ask you there that you can change your top 3 preferences here.

Before going to interview you again have a preference form (3/4 copies) to be submitted before you take ur interview.

Regards,