Billing Status C in Sales Ord not getting reversed on creating Credit Memo?

Dear SD Guru's

I have a small query, i have such a scenario.

I create sales order for a saleable service material from (01.02.08 to 29.02.08) through order type (ZOOH)

Raise invoice in advance for 30 days on 01.02.08 through billing type (ZOIN) say 100 rupees.

Now on 15th client says he doesnt want the service and i issue a credit memo request (ZOCR) against my invoice type (ZOIN) and change the contract dates from 16.02.08 to 29.02.08 for value 50 rupees.

Now i create credit memo (ZOCM) for 16.02.08 to 29.02.08.

Now once i do that i want the billing status to be reversed in my original document (ZOOH) for period 16.02.08 to 29.02.08

How can we achieve this?

Please its urgent. Points will be suitably rewarded

Regards

Ravi