Are you sure NZ6 is replaced with NZ3? NZs numbering convention doesn't normally swap even/odd around like that. NZ3/4 are the flight numbers for additional LAX flights (and were used when LAX was 3x daily) but I'd expect 6 to be replaced with 4 and 5 replaced with 3.
The issue will be related to the 789 issue and aircraft swaps that need to occur.