Hi,
As mentioned by JP, If system has to determine the new tax date your service render date should fall under that new rate condition records.
But, just want to understand onething, why do you want to apply new rate when you are creating order on 19.03.2013. When will you do the delivery and Billing?
Anyway, Go to Sales order - Header - Billing document - Billing - Service rendered date- Here try to give new date and check whats the rate system is picking up.
Let us know if you still have any problem.
Regards, Sai Krishna.