All changed at the airport by the Qantas desk - she was a very sour old lady and insisted that my handwritten coupon needed to be revalidated by the QF travel shop. Checked in today with the original coupon and had no issues at all.

On a side note, was denied access to the QF lounge as the flight is a QF codeshare and not on OW metal. After a long chap with the chap on the desk I was eventually let in!