Should payment on new calculation have been pro rata?

Child maintenance amount has gone down on the annual review by CMS due to a new job paying less money. The payments are made on the first of the month and on the paper work the new amount was from 15 December. The payment made on the 1st December was for the new lower payment. Should this have been made pro rata ie 14 days on the old amount and the rest of the month on the lower new amount? It's not much but every little helps in the lead up to Christmas. Thanks.