I have heard of NZ allowing paid upgrades in an overbook situation. The other day at premium check-in in AKL there was a sign saying ask your check in agent about a space available upgrade to premium economy. I’ve never had any experience with them however as I have always booked in business (the new one is very nice btw, nicer than UA F suites). I suggest you ask in the NZ forum, they may know better over there about options for paid upgrades since you ca definitely not do it on UA miles. Or I can ask in AKL tomorrow on the way up to SFO.