Thanks daniellam for your informative post. However I am still confused about:

1) The agent said the ticket nos will still be the same? How can this be when it is a reissue

2) The flight I am changing is on QF and not CX, so I don't understand how this will affect my CX segments if thye just changed the dates in the booking without reissuing the tix.

3) This never happened to me before!