As I understand it, for this to go through, you will need SQ's I class to be available on your particular flight. Then you will probably need SQ's I class to be also bookable by NZ which is virtually impossible. SQ often releases more I class seats to the likes of VA and AC, but never NZ (or it was NZ who blocks these seats). It's very rare that you will have all of these variables go your way.

Also, this system probably is broken anyway. A few years ago, I raised an issue about a potential AC upgrade not going through. Their response was along the line of I was one of the first customers attempting such an upgrade and they were not sure if the system was ever tested or not.