I plan to travel Chicago to Christchurch in February 2026. The SFO to CHC leg is on Air New Zealand, booked through United. I plan to purchase an Economy seat, and upgrade only the SFO to CHC to Premium Economy. Is it best to do that right after booking - or if I wait will I get a better deal? I might upgrade the return trip as well - but again, I'm perfectly content in economy from ORD to SFO and am trying to save myself a little money. I flew ORD to Belgium earlier this year and upgraded my seat while waiting at the gate. Trying to decide what's the best way to do this. Thanks for any advice.