CASA has admitted that this amendment was a balls-up. All it was supposed to do was replace and re-order the old "assigned" call, but it didn't turn out that way. They've suggested that we say this:
"MEL CENTRE, XYZ, Cleared FL190, Maintaining FL310".
Why the order change, I don't know. Perhaps ATCs can only remember one number, so it's best putting the assigned level first instead of last?!
Wait one for a change to AIP.