Clyde used to have the plates produced in batches, so its probably reasonable to assume that locomotive 2423, the last in the first batch of 2400s, serial number 78-865, had an original type plate. Serial numbers 866 to 900 inclusive were allocated to locomotives Clyde rebuilt for the NZR over a three year period. Serial numbers 866 to 869 have 77 prefixes, 870 to 893 have 78, and 894 to 900 have 79. Serial numbers 79-901 to 79-925 were allocated to a batch of locomotives built for QR. Because I can confirm that one of the locos from that batch, 2452, had a new type plate I'm assuming that the first loco from the batch, 2150, with serial number 79-902, also had the new plate.
If anyone has photos that can help narrow the gap of when the change occurred or actually knows the serial number of the first new plate a post with that information will be very much appreciated.