I've got a family member flying on Thai from BKK-NRT on an Air Canada issued ticket where the segment is an AC codeshare flight number (operated by TG). Codeshare fare class is Y (it was a flexible ticket).

As it's an AC issued ticket, my understanding is that it can't be upgraded in advance and it looks like the codeshare aspect prevents using a Star Alliance ugprade award, leaving a day of departure purchase being the only upgrade opportunity? Would this still be a possibility to buy an upgrade at check-in? Does the codeshare aspect prevent any last minute upgrade purchases? Looking at the link above, it looks like BKK-NRT would start at 13,300 TBH if the Y codeshare maps over to a TG Y? Appreciate any insight or experience folks can offer here.