Had the same change to Christchurch some weeks back. Seats were all reserved onlin for an updated 777-200 and was changed a day before departure to 777-300 with the old seating. No explanation, no new seat assignments and nobody I asked at SQ reservations, country mgr or airport staff knew why.

Their best guess : when the assigned aircrafts breakdown or are delayed, SQ obviously has a couple "standby" (= not yet 100% maxed out) ones in Changi. They are not going to park their new metal as standby, so its always likely to be an old one. There is no competition to NZ, so more likely to pull good aircraft for other routes and leave the old ones for NZ in case they are short.

Don't expect too much on the NZ route anyway. Only 1 out of 4 of my last flights downunder this year did they address passengers by name in business class.