I put upgrade Y=>PE on the waiting list of AY1963 at T-53h. At T-48h my AmexGBT app sent two alerts, one saying that my flight has been cancelled and the other one saying my flight has been confirmed. This is usually a sign of successful upgrade. My AY app continued to show 'Pending', however, until I checked in at HEL. Go figure.

I had earlier put Y=>C upgrade on the waiting list of AY1964 via WhatsApp. At T-62h I received an email saying my upgrade failed. I checked and they were still selling at least one business ticket on the flight. So, I sent a message to Plat WhatsApp, and a few minutes later my upgrade was confirmed. Nice.