Replace them with what?

That's what Alex is saying... If they replace the 733s with 737NGs, they will have to invest in NG sims etc but if they get A319/320s then they can use the current sims and training/maintenance infrastructure...
Boeing might cut them a sweet deal, take the A320's and 733, replace with 737NG, keeping in mind NZ has 77W's on the way with the option to buy more, and NZ is the launch customer for the 787-9. Maintenance costs might be reduced over time if NZ operate an all Boeing fleet also.