If the Prime Rate changes, almost all credit card agreements are written in a way that the adjustment affects cardholders’ new and existing balances without any special notice required. The exact timing of the change varies. Sometimes the issuer uses the Prime Rate on your statement date. It might also be the first day of the month or the last day of the month or another date of their choosing.The CARD Act , which took effect in 2010, altered the way credit card rates were set. Previously, credit card issue ...