R12 Invoice Tax Allocation and Reversal

We are in the processing of upgrading from 11.5.10.2  to R12. I am working on Tax allocation to Invoice lines. Things are little different in R12.

I have setup a sample tax details, created summary tax line using the tax details window for a test invoice.

I wanted to reverse the tax line, it does not allow direct reversal of tax lines, had to discard all the lines to which the tax was prorated. I had to create all the line items again and when I again try to create a summary tax line , I get a message "

"A summary tax line already exists with this combination of tax regime code, tax,tax status code and tax rate code.Please select a different combination to create the summary tax line"