Reconciliation Account change in Billing doucment

Dear All,

By mistake User assigned Reconciliation Account as 10000 and created up to Excise Invoice, this was happen November 2011, But now user realized that he has to assign Reconciliation Account as 20000, as per user requirement G/L amount need to hit Reconciliation Account 20000 for the same Billing Number. how to achieve this please suggest.

Regards

SK