Updating the TAX Code after record is saved

Hello,

I have a scenarios under which a Invoice is created  by using tax rate of 19 % . The invoice is in incomplete state.

Later I change the tax Classification code from 19% to 21 %, and save the records, System gives the message as record saved, but the details in the TAX tab is still reflecting to the Old Tax classification code which is 19% and hence the tax amount calculates is also not correct?

 This happens only with the upgraded invoices from 11i.

If we create a new invoice and update the tax classification code, it takes the correct tax % and all.

Is this  a bug or something or we have delete the whole line and manually enter a new line?