Barry Salter wrote

Providing the new ticket expires on the same date as the original
ticket, this involves calculating a "base rate" for each ticket (this
[...]

If, on the other hand, you wanted the "changeover" to be for a longer
or
shorter period than that left on the original ticket, then you would
need to apply for a refund (which would be calculated by deducting
[...]

Err, no. Longer is fine and a later than original ticket expiry may be
required.. As I quoted from the NR National Conditions of Carriage :-

However, the validity of the new ticket must start on the day after the
original ticket is handed in and must be for a period that is at least
as long as that of the original ticket when it was issued. You will not
have to pay an administrative charge. ==


--
Mike D