Your “A”category pay will start on the date of your declaration of result, i.e. 31/8/2015(Unlike high school teacher who enjoy training benefit from the date of last exam.
There is no incremental benefit(like you get for promotion) for achieving “A” category. However your grade pay will be increased from Rs. 2300 to Rs.2600(IF you have already reached or crossed “A” category pay in the pay band or your pay will be fixed at the minimum pay in the pay band for “A” category and your grade will be Rs.2600. Band pay and grade pay together makes your basic pay.