Updating a Invoice Tax Line (R12.1.3)

Hello,

I have a user who has entered an invoice whereby they entered an line for Item and entered a line for Tax.  The user saved the invoice and validated the invoice.  The invoice was put on hold due to the fact the user entered an incorrect amount for the tax.  The tax line cannot be updated at this point.  An additional line for Tax cannot be entered for the difference in what the tax should have been and what was entered; the type of Tax is no longer in the LOV.  I cannot discard the Tax line.  I can discard the Item line which