I'm flying BKK-(TG J)-SIN-(SQ J)-AKL-(NZ)-WLG tomorrow.
I can't change my NZ seat online because of prior flights in the booking.
At T-48 my SQ flight opened. I also couldn't change my seat online because first flight was on TG. A call to SQ enabled me to get a bulkhead seat that had been blocked.
I'll see what I can do at T-24 with TG.
Seamless Star Alliance!!