Here is the situation.

In Odoo, We have configured one Tax Ledger code called "IGST Account" for both Purchase and Sales Transactions. So, The Tax values for both Purchase and Sales transactions posted in same Ledger account ("IGST Account") in the past.

Now, we would like create separate Tax ledger code called "IGST-Purchase" and move all the past Purchase transactions tax values to this new account (IGST-Purchase) completely from old "IGST Account". 

How can we do that without any back end update? 

Thanks

Odoo@Tenth planet