Changing Requisition's Item Category - Tax Recalculation

We are experiencing an issue with editing the item category on a saved requisition.  If we have a taxable item category when the requisition is created, and then this is changed to a non-taxable category, the tax recalculates to zero (this is correct).  However, if we have a non-taxable category when the requisition is created, and then this is changed to a taxable category, the tax remains at zero and does not recalculate (incorrect).  How can this be corrected so tax is accurate should the item category be revised?