SAP Message AH773 - Not possible to change the rate category

Diagnosis
You changed the rate category for installation &v1& on &v2&. Aperiod-end billing has been established for this installation. Thecurrent period-end billing period starts on &v3&. You are not permittedto change the rate category within the current period-end billingperiod.

System Response

Procedure
If you wish to change the rate category, you can do so at the start ofthe next period-end billing period. You can end the period-end billingperiod early by executing a final billing.