R12 _Discard Accounted Invoice line not releasing invoice number field

Hi all

After discarding an accounted invoice line in order for the invoice number field to be ammendable(yellow), the invoice number field remains greyed out. This was not the case in R11. Is it possible to ave the invoice number ammendable for accounted invoice after discarding the line. This will allow us to change the invoice number before cancelling the invoice.

Please assist in any way possible.