SAP Message AJ678 - Error changing from rate category &1 to rate category &2

Diagnosis On &v3& you changed rate category &v1& that is billed with dynamicperiod control (DPC) to rate category &v2& that is billed without DPC.This change is only permitted at the start of billing period &v4&. System Response Processing is terminated. Procedure Change the rate category at the start of the billing period.