Subscription changes mid term

Subscription changes mid term

I created Annual Subscription X 1  for a Yearly Plan of 12 months  at  say  12000/-

now after  3 months i changed the Qty from 1 to 2  , how much will it shall charge for the remaining  9 months for the 2nd Qty - 12000 or it shall pro rata to 9000 . 

Kindly gude.