Are there changes in how banks apply prepayments to monthly interest?

For two years my prepayments were applied to interest & any principal from the 1st of the month through the prepayment date usually the 15th. For 2014, prepayments are applied to the total interest for the month which negates making a prepayment since the prepayment can't satisfy all of the interest. It seems to slow the accruing principal. Is this change by the mortgage holder legal?