I am travelling CX business class CDG-HKG in Feb and have booked CX9528 (TGV from Poitiers to CDG) and it did the same to me and booked standard class, I wanted to go first class on the TGV but there doesn't seem to be any way to upgrade it on-line. I was going to wait until checking in at Poitiers to see if I could upgrade then so will be interested if you find out anything more.